Network Working Group Y. Rekhter
Request for Comments: 2008 T. Li
BCP: 7 Cisco Systems
Category: Best Current Practice October 1996
Implications of Various Address Allocation
Policies for Internet Routing
Status of this Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
The addressing constraints described in this document are largely the
result of the interaction of existing router technology, address
assignment, and architectural history. After extensive review and
discussion, the authors of this document, the IETF working group that
reviewed it, and the IESG have concluded that there are no other
currently deployable technologies available to overcome these
limitations. In the event that routing or router technology develops
to the point that adequate routing aggregation can be achieved by
other means or that routers can deal with larger routing and more
dynamic tables, it may be appropriate to review these constraints.
IP unicast address allocation and management are essential
operational functions for the Public Internet. The exact policies for
IP unicast address allocation and management continue to be the
subject of many discussions. Such discussions cannot be pursued in a
vacuum - the participants must understand the technical issues and
implications associated with various address allocation and
The purpose of this document is to articulate certain relevant
fundamental technical issues that must be considered in formulating
unicast address allocation and management policies for the Public
Internet, and to provide recommendations with respect to these
The major focus of this document is on two possible policies,
"address ownership" and "address lending," and the technical
implications of these policies for the Public Internet. For the
organizations that could provide reachability to a sufficiently large
fraction of the total destinations in the Internet, and could express
such reachability through a single IP address prefix the document
suggests to use the "address ownership" policy. However, applying the
"address ownership" policy to every individual site or organization
that connects to the Internet results in a non-scalable routing.
Consequently, this document also recomments that the "address
lending" policy should be formally added to the set of address
allocation policies in the Public Internet. The document also
recommends that organizations that do not provide a sufficient degree
of routing information aggregation, but wish to obtain access to the
Internet routing services should be strongly encouraged to use this
policy to gain access to the services.
2 On the intrinsic value of IP addresses
Syntactically, the set of IPv4 unicast addresses is the (finite) set
of integers in the range 0x00000000 - 0xDFFFFFFF. IP addresses are
used for Network Layer (IP) routing. An IP address is the sole piece
of information about the node injected into the routing system.
The notable semantics of an IP unicast address is its ability to
interact with the Public Internet routing service and thereby
exchange data with the remainder of the Internet. In other words, for
the Public Internet, it is the reachability of an IP address that
gives it an intrinsic value. Observe, however, that IP addresses are
used outside of the Public Internet. This document does not cover the
value of addresses in other than the Public Internet context.
The above implies that in the Public Internet it is the service
environment (the Internet) and its continued operation, including its
routing system, which gives an IP address its intrinsic value, rather
than the inverse. Consequently, if the Public Internet routing system
ceases to be operational, the service disappears, and the addresses
cease to have any functional value in the Internet. At this point,
for the Public Internet, all address allocation and management
policies, including existing policies, are rendered meaningless.
3 Hierarchical routing and its implication on address allocation
Hierarchical routing [Kleinrock 77] is a mechanism that improves the
scaling properties of a routing system. It is the only proven
mechanism for scaling routing to the current size of the Internet.
Hierarchical routing requires that addresses be assigned to reflect
the actual network topology. Hierarchical routing works by taking the
set of addresses covered by a portion of the topology, and generating
a single routing advertisement (route) for the entire set. Further,
hierarchical routing allows this to be done recursively: multiple
advertisements (routes) can be combined into a single advertisement
(route). By exercising this recursion, the amount of information
necessary to provide routing can be decreased substantially.
A common example of hierarchical routing is the phone network, where
country codes, area codes, exchanges, and finally subscriber lines
are different levels in the hierarchy. In the phone network, a switch
need not keep detailed routing information about every possible
subscriber in a distant area code. Instead, the switch usually knows
one routing entry for the entire area code.
Notice that the effect on scaling is dramatic. If we look at the
space complexity of the different schemes, the switch that knows
about every subscriber in the world needs O(n) space for n worldwide
subscribers. Now consider the case of hierarchical routing. We can
break n down into the number of subscribers in the local area (l),
the other exchanges in the area code (e), the other area codes in the
local country code (a) and other country codes (c). Using this
notation, hierarchical routing has space complexity O(l + e + a + c).
Notice that each of these factors is much, much less than n, and
grows very slowly, if at all. This implies that a phone switch can be
built today that has some hope of not running out of space when it is
The fundamental property of hierarchical routing that makes this
scalability possible is the ability to form abstractions: here, the
ability to group subscribers into exchanges, area codes and country
codes. Further, such abstractions must provide useful information for
the ability to do routing. Some abstractions, such as the group of
users with green phones, are not useful when it comes time to route a
Since the information that the routing system really needs is the
location of the address within the topology, for hierarchical
routing, the useful abstraction must capture the topological location
of an address within the network. In principle this could be
accomplished in one of two ways. Either (a) constrain the topology
(and allowed topology changes) to match address assignment. Or, (b)
avoid constraints on the topology (and topology changes), but require
that as the topology changes, an entity's address change as well. The
process of changing an entity's address is known as "renumbering."
4 Scaling the Internet routing system
The enormous growth of the Public Internet places a heavy load on the
Internet routing system. Before the introduction of CIDR the growth
rate had doubled the size of the routing table roughly every nine
months. Capacity of computer technology doubles roughly every 24
months. Even if we could double the capacities of the routers in the
Internet every 24 months, inevitably the size of the routing tables
is going to exceed the limit of the routers. Therefore, to preserve
uninterrupted continuous growth of the Public Internet, deploying
mechanisms that contain the growth rate of the routing information is
Lacking mechanisms to contain the growth rate of the routing
information, the growth of the Internet would have to be either
limited or frozen, or the Internet routing system would become
overloaded. The result of overloading routing is that the routing
subsystem will fail: either equipment (routers) could not maintain
enough routes to insure global connectivity, or providers will simply
exclude certain routes to insure that other routes provide
connectivity to particular sites. This document assumes that neither
of the outcomes mentioned in this paragraph is acceptable.
Classless Inter-Domain Routing (CIDR) [RFC1518, RFC1519] has been
deployed since late 1992 in the Public Internet as the primary
mechanism to contain the growth rate of the routing information -
without CIDR the Internet routing system would have already
collapsed. For example, in October 1995, within AlterNet (one of the
major Internet Service Providers) there were 3194 routes. Thanks to
aggregation, AlterNet advertised only 799 routes to the rest of the
Internet - a saving of 2395 routes (75%) [Partan 95]. In October 1995
the Internet Routing Registry (IRR) contained 61,430 unique prefixes
listed, not counting prefixes marked as withdrawn (or 65,191 prefixes
with prefixes marked as withdrawn). That is roughly a lower bound
since many prefixes are not registered in the IRR. CIDR aggregation
resulted in less than 30,000 routes in the default-free part of the
Internet routing system [Villamizar 95].
CIDR is an example of the application of hierarchical routing in the
Public Internet, where subnets, subscribers, and finally providers
are some possible levels in the hierarchy. For example, a router
within a site need not keep detailed routing information about every
possible host in that site. Instead, the router maintains routing
information on a per subnet basis. Likewise, a router within a
provider need not keep detailed routing information about individual
subnets within its subscribers. Instead, the router could maintain
routing information on a per subscriber basis. Moreover, a router
within a provider need not keep detailed routing information about
stub (single home) subscribers of other providers by maintaining
routing information on a per provider basis.
Because of pre-CIDR address allocation, many routes in the Internet
are not suitable for hierarchical aggregation. Moreover, unconnected
sites with pre-CIDR address allocations exist. If these sites connect
to the Internet at some point in the future, the routes to these
sites are unlikely to be suitable for hierarchical aggregation. Also,
when a site uses addresses obtain from its provider, but then later
switches to a different provider (while continuing to use the same
addresses), the route to the site may no longer be suitable for
Hierarchical routing requires that aggregation boundaries for the
addressing information be formed along some hierarchy. As a result,
many exceptions will be injected into the routing system in the
future, besides those exceptions that currently exist. Each exception
added to the routing system deters the scalability of the routing
system. The exact number of exceptions that can be tolerated is
dependent on the technology used to support routing. Unbridled growth
in the number of such exceptions will cause the routing system to
5 Address allocation and management policies
IP address allocation and management policy is a complex,
multifaceted issue. It covers a broad range of issues, such as who
formulates the policies, who executes the policies, what is the role
of various registries, what is the role of various organizations
(e.g., ISOC, IAB, IESG, IETF, IEPG, various government bodies, etc.),
the participation of end users in requesting addresses, and so on.
Address allocation and management and the scalability of the routing
system are interrelated - only certain address allocation and
management policies yield scalable routing. The Internet routing
system is subject to both technological and fundamental constraints.
These constraints restrict the choices of address allocation policies
that are practical.
5.1 The "address ownership" allocation policy and its implications on
the Public Internet
"Address ownership" is one possible address allocation and management
policy. The "address ownership" policy means that part of the address
space, once allocated to an organization, remains allocated to the
organization as long as that organization wants it. Further, that
portion of the address space would not be allocated to any other
organization. Often, such addresses are called "portable." It was
assumed that if an organization acquires its addresses via the
"address ownership" policy, the organization would be able to use
these addresses to gain access to the Internet routing services,
regardless of where the organization connects to the Internet.
While it has never been explicitly stated that various Internet
Registries use the "address ownership" allocation policy, it has
always been assumed (and practiced).
To understand the implications of the "address ownership" policy
("portable" addresses) on the scalability of the Internet routing
system, one must observe that:
(a) By definition, address ownership assumes that addresses, once
assigned, fall under the control of the assignee. It is the
assignee that decides when to relinquish the ownership (although
the decision could be influenced by various factors).
Specifically, the assignee is not required (but may be influenced)
to relinquish the ownership as the connectivity of the assignee to
the Internet changes.
(b) By definition, hierarchical routing assumes that addresses
reflect the network topology as much as possible.
Therefore, the only presently known practical way to satisfy both
scalable hierarchical routing and address ownership for everyone is
to assume that the topology (or at least certain pieces of it) will
be permanently fixed. Given the distributed, decentralized, largely
unregulated, and global (international) nature of the Internet,
constraining the Internet topology (or even certain parts of it) may
have broad technical, social, economical, and political implications.
To date, little is known of what these implications are; even less is
known whether these implications would be acceptable (feasible) in
practice. Therefore, at least for now, we have to support an Internet
with an unconstrained topology (and unconstrained topological
Since the Internet does not constrain its topology (or allowed
topology changes), we can either have address ownership for everyone
or a routable Internet, but not both, or we need to develop and
deploy new mechanisms (e.g., by decoupling the address owned by the
end users from those used by the Internet routing, and provide
mechanisms to translate between the two). In the absence of new
mechanisms, if we have address ownership ("portable" addresses) for
everyone, then the routing overhead will lead to a breakdown of the
routing system resulting in a fragmented (partitioned) Internet.
Alternately, we can have a routable Internet, but without address
ownership ("portable" addresses) for everyone.
5.2 The "address lending" allocation policy and its implications for the
Recently, especially since the arrival of CIDR, some subscribers and
providers have followed a model in which address space is not owned
(not portable), but is bound to the topology. This model suggests an
address allocation and management policy that differs from the
"address ownership" policy. The following describes a policy, called
"address lending," that provides a better match (as compared to the
"address ownership" policy) to the model.
An "address lending" policy means that an organization gets its
addresses on a "loan" basis. For the length of the loan, the lender
cannot lend the addresses to any other borrower. Assignments and
allocations based on the "address lending" policy should explicitly
include the conditions of the loan. Such conditions must specify that
allocations are returned if the borrower is no longer contractually
bound to the lender, and the lender can no longer provide aggregation
for the allocation. If a loan ends, the organization can no longer
use the borrowed addresses, and therefore must get new addresses and
renumber to use them. The "address lending" policy does not constrain
how the new addresses could be acquired.
This document expects that the "address lending" policy would be used
primarily by Internet Registries associated with providers; however,
this document does not preclude the use of the "address lending"
policy by an Internet Registry that is not associated with a
This document expects that when the "address lending" policy is used
by an Internet Registry associated with a provider, the provider is
responsible for arranging aggregation of these addresses to a degree
that is sufficient to achieve Internet-wide IP connectivity.
This document expects that when the "address lending" policy is used
by an Internet Registry associated with a provider, the terms and
conditions of the loan would be coupled to the service agreement
between the provider and the subscribers. That is, if the subscriber
moves to another provider, the loan would be canceled.
To reduce disruptions when a subscriber changes its providers, this
document strongly recommends that the terms and conditions of the
loan should include provision for a grace period. This provision
would allow a subscriber that disconnects from its provider a certain
grace period after the disconnection. During this grace period, the
borrower (the subscriber) may continue to use the addresses obtained
under the loan. This document recommends a grace period of at least
30 days. Further, to contain the routing information overhead, this
document suggests that a grace period be no longer than six months.
To understand the scalability implications of the "address lending"
policy, observe that if a subscriber borrows its addresses from its
provider's block, then the provider can advertise a single address
prefix. This reduces the routing information that needs to be carried
by the Internet routing system (for more information, see Section
5.3.1 of RFC1518). As the subscriber changes its provider, the loan
from the old provider would be returned, and the loan from the new
provider would be established. As a result, the subscriber would
renumber to the new addresses. Once the subscriber renumbers into the
new provider's existing blocks, no new routes need to be introduced
into the routing system.
Therefore, the "address lending" policy, if applied appropriately, is
consistent with the constraints on address allocation policies
imposed by hierarchical routing, and thus promotes a scalable routing
system. Thus, the "address lending" policy, if applied
appropriately, could play an important role in enabling the
continuous uninterrupted growth of the Internet.
To be able to scale routing in other parts of the hierarchy, the
"lending" policy may also be applied hierarchically, so that
addresses may in turn be lent to other organizations. The implication
here is that the end of a single loan may have effects on
organizations that have recursively borrowed parts of the address
space from the main allocation. In this case, the exact effects are
difficult to determine a priori.
5.3 In the absence of an explicit "address lending" policy
Organizations connecting to the Internet should be aware that even if
their current provider, and the provider they switch to in the future
do not require renumbering, renumbering may still be needed to
achieve Internet-wide IP connectivity. For example, an organization
may now receive Internet service from some provider and allocate its
addresses out of the CIDR block associated with the provider. Later
the organization may switch to another provider. The previous
provider may still be willing to allow the organization to retain
part of the provider's CIDR block, and accept a more specific prefix
for that organization from the new provider. Likewise, the new
provider may be willing to accept that organization without
renumbering and advertise the more specific prefix (that covers
destinations within the organization) to the rest of the Internet.
However, if one or more other providers exist, that are unwilling or
unable to accept the longer prefix advertised by the new provider,
then the organization would not have IP connectivity to part of the
Internet. Among the possible solutions open to the organization may
be either to renumber, or for others to acquire connectivity to
providers that are willing and able to accept the prefix.
The above shows that the absence of an explicit "address lending"
policy from a current provider in no way ensures that renumbering
will not be required in the future when changing providers.
Organizations should be aware of this fact should they encounter a
provider making claims to the contrary.
Observe that the goal of hierarchical routing in the Internet is not
to reduce the total amount of routing information in the Internet to
the theoretically possible minimum, but just to contain the volume of
routing information within the limits of technology,
price/performance, and human factors. Therefore, organizations that
could provide reachability to a sufficiently large fraction of the
total destinations in the Internet and could express such
reachability through a single IP address prefix could expect that a
route with this prefix will be maintained throughout the default-free
part of the Internet routing system, regardless of where they connect
to the Internet. Therefore, using the "address ownership" policy
when allocating addresses to such organizations is a reasonable
choice. Within such organizations this document suggests the use of
the "address lending" policy.
For all other organizations that expect Internet-wide IP
connectivity, the reachability information they inject into the
Internet routing system should be subject to hierarchical
aggregation. For such organizations, allocating addresses based on
the "address ownership" policy makes hierarchical aggregation
difficult, if not impossible. This, in turn, has a very detrimental
effect on the Internet routing system. To prevent the collapse of the
Internet routing system, for such organizations, this document
recommends using the "address lending" policy. Consequently, when
such an organization first connects to the Public Internet or changes
its topological attachment to the Public Internet, the organization
eventually needs to renumber. Renumbering allows the organization to
withdraw any exceptional prefixes that the organization would
otherwise inject into the Internet routing system. This applies to
the case where the organization takes its addresses out of its direct
provider's block and the organization changes its direct provider.
This may also apply to the case where the organization takes its
addresses out of its indirect provider's block, and the organization
changes its indirect provider, or the organization's direct provider
changes its provider.
Carrying routing information has a cost associated with it. This
cost, at some point, may be passed back in full to the organizations
that inject the routing information. Aggregation of addressing
information (via CIDR) could reduce the cost, as it allows an
increase in the number of destinations covered by a single route.
Organizations whose addresses are allocated based on the "address
ownership" policy (and thus may not be suitable for aggregation)
should be prepared to absorb the cost completely on their own.
Observe that neither the "address ownership," nor the "address
lending" policy, by itself, is sufficient to guarantee Internet-wide
IP connectivity. Therefore, we recommend that sites with addresses
allocated based on either policy should consult their providers about
the reachability scope that could be achieved with these addresses,
and associated costs that result from using these addresses.
If an organization doesn't require Internet-wide IP connectivity,
then address allocation for the organization could be done based on
the "address ownership" policy. Here, the organization may still
maintain limited IP connectivity (e.g., with all the subscribers of
its direct provider) by limiting the distribution scope of its
routing information to its direct provider. Connectivity to the rest
of the Internet can be handled by mediating gateways (e.g.,
application layer gateways, Network Address Translators (NATs)). Note
that use of mediating gateways eliminates the need for renumbering,
and avoids burdening the Internet routing system with non-
aggregatable addressing information; however they have other
drawbacks which may prove awkward in certain situations.
Both renumbering (due to the "address lending" policy), and non-
aggregated routing information (due to the "address ownership"
policy), and the use of mediating gateways result in some costs.
Therefore, an organization needs to analyze its own connectivity
requirements carefully and compare the tradeoffs associated with
addresses acquired via either policy vs. having connectivity via
mediating gateways (possibly augmented by limited IP connectivity)
using addresses acquired via "address ownership." To reduce the cost
of renumbering, organizations should be strongly encouraged to deploy
tools that simplify renumbering (e.g., Dynamic Host Configuration
Protocol [RFC 1541]). Use of the DNS should be strongly encouraged.
Any address allocation and management policy for IP addresses used
for Internet connectivity must take into account its impact on the
scalability of the Public Internet routing system. Among all of the
possible address allocation and management policies only the ones
that yield a scalable routing system are feasible. All other policies
are self-destructive in nature, as they lead to a collapse of the
Internet routing system, and therefore to the fragmentation
(partitioning) of the Public Internet.
Within the context of the current Public Internet, address allocation
and management policies that assume unrestricted address ownership
have an extremely negative impact on the scalability of the Internet
routing system. Such policies are almost certain to exhaust the
scalability of the Internet routing system well before we approach
the exhaustion of the IPv4 address space and before we can make
effective use of the IPv6 address space. Given the Internet's growth
rate and current technology, the notion that everyone can own address
space and receive Internet-wide routing services, despite where they
connect to the Internet, is currently technically infeasible.
Therefore, this document makes two recommendations. First, the
"address lending" policy should be formally added to the set of
address allocation policies in the Public Internet. Second,
organizations that do not provide a sufficient degree of routing
information aggregation to obtain access to the Internet routing
services should be strongly encouraged to use this policy to gain
access to the services.
Since the current IPv6 address allocation architecture is based on
CIDR, recommendations presented in this document apply to IPv6
address allocation and management policies as well.
8 Security Considerations
Renumbering a site has several possible implications on the security
policies of both the site itself and sites that regularly communicate
with the renumbering sites.
Many sites currently use "firewall" systems to provide coarse-grained
access control from external networks, such as The Internet, to their
internal systems. Such firewalls might include access control
decisions based on the claimed source address of packets arriving at
such firewall systems. When the firewall policy relates to packets
arriving on the firewall from inside the site, then that firewall
will need to be reconfigured at the same time that the site itself
renumbers. When the firewall policy relates to packets arriving at
the firewall from outside the site, then such firewalls will need to
be reconfigured whenever an outside site that is granted any access
inside the site through the firewall is renumbered.
It is highly inadvisable to rely upon unauthenticated source or
destination IP addresses for security policy decisions. [Bellovin89]
IP address spoofing is not difficult with widely available systems,
such as personal computers. A better approach would probably involve
the use of IP Security techniques, such as the IP Authentication
Header [RFC-1826] or IP Encapsulating Security Payload [RFC-1827], at
the firewall so that the firewall can rely on cryptographic
techniques for identification when making its security policy
It is strongly desirable that authentication be present in any
mechanism used to renumber IP nodes. A renumbering mechanism that
lacks authentication could be used by an adversary to renumber
systems that should not have been renumbered, for example.
There may be other security considerations that are not covered in
This document borrows heavily from various postings on various
mailing lists. Special thanks to Noel Chiappa, Dennis Ferguson, Eric
Fleischman, Geoff Huston, and Jon Postel whose postings were used in
Most of the Section 5.3 was contributed by Curtis Villamizar. The
Security section was contributed by Ran Atkinson.
Many thanks to Scott Bradner, Randy Bush, Brian Carpenter, Noel
Chiappa, David Conrad, John Curran, Sean Doran, Dorian Kim, Thomas
Narten, Andrew Partan, Dave Piscitello, Simon Poole, Curtis
Villamizar, and Nicolas Williams for their review, comments, and
contributions to this document.
Finally, we like to thank all the members of the CIDR Working Group
for their review and comments.
[Bellovin89] Bellovin, S., "Security Problems in the TCP/IP Protocol
Suite", ACM Computer Communications Review, Vol. 19, No. 2, March
[Kleinrock 77] Kleinrock, L., and K. Farouk, K., "Hierarchical
Routing for Large Networks," Computer Networks 1 (1977), North-
Holland Publishing Company.
[Partan 95] Partan, A., private communications, October 1995.
[RFC 1541] Droms, R., "Dynamic Host Configuration Protocol", October
[RFC 1519] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless
Inter-Domain Routing (CIDR): an Address Assignment and Aggregation
Strategy", September 1993.
[RFC 1518] Rekhter, Y., and T. Li, "An Architecture for IP Address
Allocation with CIDR", September 1993.
[RFC 1825] Atkinson, R., "IP Security Architecture", RFC 1825, August
[RFC 1826] Atkinson, R., "IP Authentication Header (AH), RFC 1826,
[RFC 1827] Atkinson, R., "IP Encapsulating Security Payload (ESP)",
RFC 1827, August 1995.
[Villamizar 95] Villamizar, C., private communications, October 1995.
10 Authors' Addresses
cisco Systems, Inc.
170 Tasman Dr.
San Jose, CA 95134
Phone: (914) 528-0090
cisco Systems, Inc.
170 Tasman Dr.
San Jose, CA 95134
Phone: (408) 526-8186