TCP/IP Addressing
To understand networking with Linux, you need to know the basics of TCP/IP
addressing. Internet IP addresses (also known as public IP addresses) are
different from those used internally on a local area network (LAN). Internet IP
addresses are assigned (for the United States and some other hosts) by the
American Registry for Internet Numbers (ARIN; see www.arin.net). Entities
that need Internet addresses apply to this agency to be assigned addresses.
ARIN assigns Internet service providers (ISPs) one or more blocks of IP
addresses, which the ISPs can then assign to their subscribers.
You will quickly recognize the current form of TCP/IP addressing, known as
IP version 4 (IPv4). In this method, a TCP/IP address is expressed as a series
of four decimal numbers: a 32-bit value expressed in a format known as
dotted-decimal format, such as 192.168.0.1. Each set of numbers is
known as an octet (eight 1s and 0s, such as 10000000 to represent 128 ) and
ranges from 0 to 255.
The first octet usually determines what class the network belongs to. There
are three classes of networks:
Class A—Consists of networks with the first octet ranging from 1 to 126.
There are only 126 Class A networks, each composed of up to
16,777,214 hosts. (If you are doing the math, there are potentially
16,777,216 addresses, but no host portion of an address can be all 0s or
255s.) The 10. network is reserved for local network use, and the 127.
network is reserved for the loopback address, 127.0.0.1. TCP/IP uses
loopback addressing to enable Linux network-related client and server
programs to communicate on the same host. This address does not
appear and is not accessible on your LAN.
NOTE
Notice that 0 is not included in Class A. The 0 address is used for network-
to-network broadcasts. Also note that there are two other classes of
networks, Classes D and E. Class D networks are reserved for multicast
addresses and are not for use by network hosts. Class E addresses are
deemed experimental and thus are not open for public addressing.
Class B—Consists of networks defined by the first two octets, with the
first ranging from 128 to 191. The 128. network is also reserved for local
network use. There are 16,382 Class B networks, each with 65,534
possible hosts.
Class C—Consists of a network defined by the first three octets with the
first ranging from 192 to 223. The 192. network is another that is