Understanding Networking Protocols
assignment to nodes. The remaining 253 unique addresses are available for assignment
in each octet.
Addresses on the Internet are guaranteed to be unique through the use of an
address registration service, presently administered by the Internet Corporation for
Assigned Names and Numbers (ICANN). Actual registrations of domain names and
addresses are handled through one of many registrars, which include companies such
as InterNIC, Network Solutions, and many others. ICANN is the overall authority.
ICANN assigns three major classes of addresses, called Class A, B, and C, as follows:
For a Class A address, ICANN assigns the owner a number in the first octet.
The owner is then free to use all possible valid combinations in the remaining
three octets. For example, a Class A address might be 57.xxx.xxx.xxx. Class A
addresses enable the owner to address up to around 16.5 million unique nodes.
Class B addresses define the first two octets, leaving the remaining two open
for the address's owner to use. For instance, 223.55.xxx.xxx would be a valid
Class B address assignment. Class B addresses enable the holder to have about
65,000 unique nodes.
Class C follows this progression, defining the first three octets and leaving only
the last octet available for the Class C owner to assign. The owner can assign
up to 255 unique addresses.
An Internet service provider (ISP) might own either a Class A or a Class B address,
and then can handle a number of Class C addresses within its own address structure.
Changing ISPs, even for a company that has a valid Class C address, means changing
the company's address from a Class C address available through the first ISP to a Class C
address available from the new ISP.
As mentioned earlier, the addresses 0, 127, and 255 are reserved. Usually, address
0--as in 184.108.40.206--refers to the network itself, and the router that connects the
network to other networks handles this address. The address 127 is a special loopback
address that can be used for certain kinds of testing. The address 255 refers to all
Help! We're Almost Out of Addresses!
The current implementation of IP, called IP version 4 (IPv4), is approaching the
point where running out of addresses is becoming a real possibility. In 1994, a
proposal was issued to address this limitation. Called IP Next Generation (IPng,
now IP version 6, or IPv6), the new version of IP takes care of the addressing
limitation by bumping up the address length from 32 bits to 128 bits. This
allows 3.4 × 10
(34 followed by 37 zeros, or around 340 trillion, trillion, trillion)
unique addresses, which should leave plenty of room for all anticipated Internet
addresses, even allowing for refrigerators, toasters, and cars to have their own IP