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 hostmaster@sri-nic.arpa 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
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(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
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and operating systems vary in the problems they have dealing
with subnets. Frequently these machines can be used as a
leaf on a network but not as a gateway within the subnetted
portion of the network. As time passes and more systems
become 4.3BSD based, these problems should disappear.
There has been some confusion in the past over the format of
an IP broadcast address. Some machines used an address of
all zeros to mean broadcast and some all ones. This was
confusing when machines of both type were connected to the
same network. The broadcast address of all ones has been
adopted to end the grief. Some systems (e.g. 4.2 BSD) allow
one to choose the format of the broadcast address. If a
system does allow this choice, care should be taken that the
all ones format is chosen. (This is explained in RFC-1009
and RFC-1010).