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How Big is the Internet?
Michael F. Schwartz
University of Colorado - Boulder
Published in Internet Society News 1(2), Spring 1992
The question often arises, "How big is the Internet?" To answer this question, we must first define what we
wish to measure. At one time, connectivity via the IP protocol suite defined the Internet. Since a number
of protocols now coexist on the Internet, some people have suggested defining the Internet instead by a
common name space (perhaps the Domain Naming System or X.500). This definition is counterintuitive,
since it elides differences between various types of physical connectivity. In particular, it does not distin-
guish the parts of the network that can support interactive applications (like remote login) from dialup-
based, mail-only connections. Given the advantages of interactive connectivity and the growing popularity
of IP, in this article I consider only the interconnected IP Internet.
Lottor recently published results of a ten year study that counted the number of hosts in domains that have
IP addresses registered in the DNS (as opposed to domains that register only "mail exchange" (MX)
records that allow mail to be forwarded to through an intermediary host) [Lottor 1992]. In the early years
the data were extracted from host tables maintained by the DDN Network Information Center. Later,
measurements were taken by a program that recursively descends the Domain Naming tree, retrieving in-
formation about all domains that allow "zone transfers".
Many of the hosts counted by Lottor's study are hidden behind secure gateways or otherwise not directly
connected to the Internet. Therefore, Lottor's study really indicates the spread of IP and the Domain Nam-
ing System at sites connected to the Internet. I believe a more meaningful measure of Internet size is the
number of domains at which common network services can be contacted, since it is through such services
that a site gains the advantages of connectivity.
I am currently performing such a study. Specifically, this study tracks changes in service-level reachability
in the Internet [Schwartz 1991]. While the measurements will not be complete until the end of 1992, the
first set of measurements that have been collected can be used to characterize the current size of the inter-
connected IP Internet. The final study will provide much more information than just Internet size. It will
indicate relative growth rates among different countries, trends in the types of services to which sites limit
access, how sites limit access to these services, and the types and geographical distribution of sites that dis-
tance themselves from the Internet.
Starting with a large list of domains, my study attempts to connect to the following TCP/IP services at each
domain:
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
Port Number Service Port Number Service iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
13 daytime 111 Sun portmap
15 netstat 513 rlogin
21 FTP 514 rsh
23 telnet 540 UUCP
25 SMTP 543 klogin
53 Domain Naming System 544 krcmd, kshell
79 finger
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This list was chosen to span a representative range of service types, each of which can be expected to be
found on any machine in a site (so that probing random machines is meaningful). The one exception is the
Domain Naming System, for which the machines to probe are selected from information obtained from the
Domain system itself. Only TCP services are tested, since the TCP connection mechanism allows one to
determine if a server is running in an application-independent fashion.
From a list of approximately 12,700 Internet domains worldwide (generated from Lottor's January 1991
data plus a number of other sources), successful connections were recorded to at least one of the above ser-
vices in 4,455 domains, broken down by top-level domain as follows:
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
Top-level Description Number of Domains Reachable by
Domain Name Measured Internet Services iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
edu U.S. Educational 2048
com U.S. Commercial 494
ca Canadian 299
au Australian 278
de German 174
se Swedish 167
gov U.S. Government 128
mil U.S. Military 115
jp Japanese 106
net Named by network 96
nl Dutch 84
org Non-profit 56
fr French 55
no Norwegian 55
fi Finnish 45
uk British 44
it Italian 39
dk Danish 38
at Austrian 21
nz New Zealand 21
ch Swiss 20
il Israeli 16
is Icelandic 8
es Spanish 8
kr Korean 5
be Belgian 4
gr Greek 4
za South African 4
br Brazil 3
ie Irish 3
tw Taiwanese 3
us Other U.S. 3
arpa ARPANET names 2
mx Mexican 2
sg Singapore 2
hk Honk Kong 1
in Indian 1
int International 1
pt Portuguese 1
tn Tunisian 1
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This list is a lower bound, since it depends on the span of the initial list of domains, and sites in other coun-
tries have connected to the Internet since this list was compiled. Nonetheless, the measurements provide an
interesting point of comparison. For example, it is clear that the number of U.S. sites is much larger than
the number of sites in any other country in the world. In fact, there are nearly twice as many U.S. sites as
sites in all other countries combined. However, given the rapid growth rate of IP connectivity in other
countries, within one to two years I expect there to be more sites internationally than in the U.S.
To help underscore the distinction between service-level connectivity and IP host count at Internet sites, I
found that 7,242 domains in Lottor's January 1991 list (out of 11,194 in that list) were not reachable by the
above Internet services. The ratio of service reachable to all IP domains may continue to decrease, as secu-
rity problems garner increasing concern. The results of my study will help uncover the trend here.
The services reached by my measurement software were as follows: iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
Service Number of Domains
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
telnet 4170
FTP 4027
SMTP 3952
rlogin 3811
rsh 3777
finger 3637
daytime 3492
Sun portmap 3421
UUCP 2217
Domain 1803
netstat 294
klogin 95
krcmd, kshell 93
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From this list it is clear that the "Big Three" applications (remote login, file transfer, and mail) are the main
services in use. Interestingly, UUCP appears in more domains than DNS, even though TCP based UUCP
(as opposed to dialup UUCP) is being phased out of existence, as NNTP gains popularity. The reason for
this is probably two fold. First, most domains contract DNS service from other domains, to avoid the
administrative effort required to run a Domain server. Second, many computers probably come with
UUCP configured in by the manufacturer.
For a discussion of the size of the set of computer networks interconnected for at least mail or news service
(referred to as "The Matrix"), see [Quarterman 1992]. For a measure of the diameter of the interpersonal
communication graph enabled by electronic mail, see [Schwartz & Wood 1992]. Anyone who is consider-
ing performing measurement studies of the Internet is urged to read [Cerf 1991].
References
[Cerf 1991]
V. G. Cerf, editor. Guidelines for Internet Measurement Activities. Req. For Com. 1262, Internet
Activities Board, Oct. 1991.
[Lottor 1992]
M. Lottor. Internet Growth (1981-1991). Req. For Com. 1296, Network Information Systems
Center, SRI Int., Jan. 1992.
[Quarterman 1992]
J. S. Quarterman. How Big is the Matrix? Matrix News, 2(2), Matrix Information and Directory
Services, Austin, TX, mids@tic.com, Feb. 1992.
[Schwartz 1991]
M. F. Schwartz. A Measurement Study of Changes in Service-Level Reachability in the Global
TCP/IP Internet: Goals, Experimental Design, Implementation, and Policy Considerations. Req. For
Com. 1273, Dept. Comput. Sci., Univ. Colorado, Boulder, CO, Nov. 1991.
[Schwartz & Wood 1992]
M. F. Schwartz and D. C. M. Wood. Discovering Shared Interests Among People Using Graph
Analysis of Global Electronic Mail Traffic. Dept. Comput. Sci., Univ. Colorado, Boulder, CO, Feb.
1992. Submitted for publication.