Network Working Group S. Bellovin
Request for Comments: 1681 AT&T Bell Laboratories
Category: Informational August 1994
On Many Addresses per Host
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
This document was submitted to the IETF IPng area in response to RFC
1550. Publication of this document does not imply acceptance by the
IPng area of any ideas expressed within. Comments should be
submitted to the email@example.com mailing list.
Overview and Rational
Currently, most hosts have only one address. With comparatively rare
exceptions, hosts as hosts -- as opposed to hosts acting as routers
or PPP servers -- are single-homed. Our address space calculations
reflect this; we are assuming that we can estimate the size of the
address space by counting hosts. But this may be a serious error. I
suggest that that model may -- and should -- change.
For the ideas outlined below, I do not claim that multiple addresses
per host is the only or even necessarily the best way to accomplish
the goal. I do claim that my ideas are at the very least plausible,
and that I expect that many of them will be tried.
More and more often, services are being encoded in the host name.
One can fetch files from ftp.research.att.com, look up an IP address
on ns.uu.net, synchronize clocks from ntp.udel.edu, etc. Should this
practice be generalized to the IP address domain?
In some cases it would be a very good idea. Certain services need to
be configured by IP address; they are either used when the DNS is
being bootstrapped (such as in glue records and root server cache
records), or when its unavailable (i.e., when booting after a power
hit, and the local name servers are slower to reboot than their
Security is another reason, in some cases. Address-based
authentication is bad enough; relying on the name service adds
another layer of risk. An attacker can go after the DNS, in that
case. A risk-averse system manager might prefer to avoid the extra
exposure, instead granting privileges (i.e., rlogin or NFS) by
address instead of name. But that, of course, leads to all the usual
headaches when the location of the service changes. If the address
for the service could be held constant, there would be much more
freedom to move it to another machine. One way to do that is by
assigning the serving host a secondary address.
A related notion comes from the need to offer different views of a
service from a single host. For example, research.att.com has long
offered two distinct FTP archives, with slightly different access
policies. It would be nice if both could live on the same machine,
without asking the user community to learn new protocols or custom
Archie is an even better example. There are three principal ways to
use Archie: use a special protocol, and hence a special application
program, on a dedicated port and host that is probably named
archie.foo.bar; telnet to archie.foo.bar and go through an extra and
gratuitous login as archie, or telnet to some special port on
archie.foo.bar. The latter two are examples of using a standard
protocol (telnet) to offer a different service. Neither alternative
is very convenient.
It would be better if archie.foo.bar provided the Archie service,
while host.foo.bar provided a login prompt. Again -- an easy way to
do this is to assign the host a separate IP address for its extra
Note that there are security advantages here, too. A firewall could
be configured to allow access to the address associated with the
Archie server, but not the other addresses on that host. That would
provide a high degree of safety, assuming, of course, that the other
servers on that host were bound to its primary addresses, and not the
Another way to implement this concept would be to extend the DNS, to
return port number information as well as IP addresses. Thus,
netlib.att.com might return 22.214.171.124/221. But that would
necessitate changing every FTP client program, a daunting task.
We could also look on this as the extension of the MX concept. MX
records are very valuable, but they apply only to mail, and they
don't supply port numbers. Again, changing this would require
massive client program changes.
Accounting and Billing
For better or worse, some parts of the Internet are moving towards
usage-sensitive charging. At least four charging schemes seem
possible; doubtless, the marketeers in charge of such things can and
will come up with more.
The first is the traditional "pay as you go" approach. Each host is
responsible for its own packets. Of course, that means that in a
typical conversation, both parties pay -- and the providers of free
FTP archives will end up paying dearly for their beneficence. That
leads to our second model: caller pays. Other people might want to
make collect calls, much as is done on the telephone today. Finally,
there might be the equivalent of American "900" numbers: the caller
pays a premium to the server.
This is not at all far-fetched; UUNET already has a 900 number for
anonymous uucp clients. No need to register in advance; just dial
in, and let the phone company act as your agent.
Given all these schemes, it is vital that the caller and recipient
know in advance who will pay. It is not acceptable for users to
learn, only after the fact, that they have incurred a cost. We could
envision use of IP options, but again, that would preclude use of
today's standard clients.
It is not sufficient to present a message at connection time warning
of the charges. Many interactions do not provide a hook for user
interaction. And there are security concerns -- suppose that someone
puts up a gopher server that redirects a caller to some pay-to-play
address, without displaying the required warning. A scam? Sure --
but it's already happened with the phone network, and I see no reason
to think that the Internet will be far behind.
My suggestion, of course, is to encode the charge algorithm in the
destination address (and perhaps in the DNS name space as well). The
bits themselves would determine who pays. Organizational border
routers could implement policies on pay services; the anonymous
workstations in a dorm computer lab wouldn't be allowed to call
An extension of this scheme would use a comparatively large number of
bits, letting the address act not just as a policy indicator, but
also as an index to a charge algorithm table.
Addresses per User
It may be useful to assign each user on a host a separate IP address,
for the duration of the login session. This has a number of
The first ties in with the charging scheme given above. Usage-
sensitive accounting today is done by routers, and they have no
notion of who is using the hosts. If each user had a separate IP
address, we could continue to gather the accounting data at the
router. The host would simply have to record the address
assignments; billing could be done offline.
Similarly, different classes of users could have different forms of
addresses. Those with hard-money accounts might have some bits set
in the address that would allow for access to costly services. The
border routers could make this sort of distinction, using today's
An IP address per user also fits in well with encryption. There is a
lot of attention today focused on network-layer encryption. But that
provides host-level granularity of protection, which is sometimes
insufficient. Transport-layer encryptors provide finer-grained
protection, but does the Internet need two different low-level
encryption schemes? If each user had a separate IP address -- and
perhaps had it only on hosts that cared about such matters -- we
could provide user-level protection and accounability, with the same
infrastructure used to support host-level accountability.
There are several schemes under discussion for mobile IP hosts.
These are aimed at a fairly general model of hosts moving anywhere.
While that is important, there is also some need for limited
mobility, within a subnet. This could be used for load-balancing. A
mail relay that had just been asked to send a large message to a huge
mailing list could offload some of its IP addresses to its peers.
That would divert future incoming messages without invalidating
thousands of cached MX records and their associated IP addresses.
Similarly, servers for low-speed X terminals could reside on
different physical machines, all the while not disturbing sessions in
There has long been some need to merge subnets. Sometimes this is
due to organizational changes; other times, people have installed
bridges when routers would have been a more appropriate choice. Some
hosts need to live on both logical networks at once, to avoid an
extra hop through a router. It would be useful to be able to assign
them such addresses.
How Many Addresses Do We Need?
Assuming that some of these ideas bear fruit, how many addresses do
we need, per host?
Most of these schemes are fairly cheap. Few people would offer more
than a handful of distinct service views per system. But the
address-per-user notion could be quite costly. We also have to
account for address mask assignment policies. In many of today's
networks, enough bits of host address have to be allocated to allow
for the largest subnet in an organization. Even if we assume that
IPng's routing protocols will be smarter about such things, foresight
in address allocation will be needed to allow headroom for some
networks to grow, while still maintaining a contiguous netmask. This
in turn will contribute to sparse utilization of the address space.
Accordingly, I recommend that we allow for 2^6, and perhaps as many
as 2^8, extra addresses per host, to leave room for the ideas
I should note that the idea of encoding the service in the transport
address bears some relation to OSI's model. That similarity should
not, of course, invalidate the idea.
Some of these ideas were derived from conversations with Matt Blaze.
Security issues are discussed throughout this memo.
Steven M. Bellovin
Software Engineering Research Department
AT&T Bell Laboratories
600 Mountain Avenue
Murray Hill, NJ 07974, USA
Phone: +1 908-582-5886
Fax: +1 908-582-3063