Hitchhikers Guide to the Internet | Page 5

day interest in the news of the NSFnet (the backbone, regional

network, and Internet inter-connection site workers). The messages are
reflected by a central location and are sent as separate messages to each
subscriber. This creates hundreds of messages on the wide area
networks where bandwidth is the scarcest.
There are two ways in which a campus could spread the news and not
cause these messages to inundate the wide area networks. One is to
re-reflect the message on the campus. That is, set up a reflector on a
local machine which forwards the message to a campus distribution list.
The other is to create an alias on a campus machine which places the
messages into a notesfile on the topic. Campus users who want the
information could access the notesfile and see the messages that have
been sent since their last access. One might also elect to have the
campus wide area network liaison screen the messages in either case
and only forward those which are considered of merit. Either of these
schemes allows one message to be sent to the campus, while allowing
wide distribution within.
Address Allocation
Before a local network can be connected to the Internet it must be
allocated a unique IP address. These addresses are allocated by ISI. The
allocation process consists of getting an application form received from
ISI. (Send a message to [email protected] and ask for the
template for a connected address). This template is filled out and
mailed back to hostmaster. An address is allocated and e-mailed back
to you. This can also be done by postal mail (Appendix B).
IP addresses are 32 bits long. It is usually written as four decimal
numbers separated by periods (e.g., 192.17.5.100). Each number is the
value of an octet of the 32 bits. It was seen from the beginning that
some networks might choose to organize themselves as very flat (one
net with a lot of nodes) and some might organize hierarchically
-6-
(many interconnected nets with fewer nodes each and a backbone). To
provide for these cases, addresses were differentiated into class A, B,
and C networks. This classification had to with the interpretation of the
octets. Class A networks have the first octet as a network address and
the remaining three as a host address on that network. Class C
addresses have three octets of network address and one of host. Class B
is split two and two. Therefore, there is an address space for a few large

nets, a reasonable number of medium nets and a large number of small
nets. The top two bits in the first octet are coded to tell the address
format. All of the class A nets have been allocated. So one has to
choose between Class B and Class C when placing an order. (There are
also class D (Multicast) and E (Experimental) formats. Multicast
addresses will likely come into greater use in the near future, but are
not frequently used now).
In the past sites requiring multiple network addresses requested
multiple discrete addresses (usually Class C). This was done because
much of the software available (not ably 4.2BSD) could not deal with
subnetted addresses. Information on how to reach a particular network
(routing information) must be stored in Internet gateways and packet
switches. Some of these nodes have a limited capability to store and
exchange routing information (limited to about 300 networks).
Therefore, it is suggested that any campus announce (make known to
the Internet) no more than two discrete network numbers.
If a campus expects to be constrained by this, it should consider
subnetting. Subnetting (RFC-932) allows one to announce one address
to the Internet and use a set of addresses on the campus. Basically, one
defines a mask which allows the network to differentiate between the
network portion and host portion of the address. By using a different
mask on the Internet and the campus, the address can be interpreted in
multiple ways. For example, if a campus requires two networks
internally and has the 32,000 addresses beginning 128.174.X.X (a
Class B address) allocated to it, the campus could allocate 128.174.5.X
to one part of campus and 128.174.10.X to another. By advertising
128.174 to the Internet with a subnet mask of FF.FF.00.00, the Internet
would treat these two addresses as one. Within the campus a mask of
FF.FF.FF.00 would be used, allowing the campus to treat the addresses
as separate entities. (In reality you don't pass the subnet mask of
FF.FF.00.00 to the Internet, the octet meaning is implicit in its being a
class B address). A word of warning is necessary. Not all systems know
how to do subnetting. Some 4.2BSD systems require additional
software. 4.3BSD systems subnet as released. Other devices
-7-
and operating systems vary in the problems they have