User`s guide

180 Appendix E Understanding IP Addresses and Subnet Masking
E.2.1 Class A Network Addresses
Class A network addresses are used for very large networks, such as those in major international
companies. Every Class A network address has an 8-bit network prefix, with the highest order bit
set to 0, followed by a seven-bit network number and a 24-bit host-number that identifies each
host. IP addresses with a first octet from 1 to 126 are part of this class. Class A networks are also
referred to as “/8s” since they have an 8-bit network prefix. The
Class A example in Figure E-1 has a major network address of 10. The next 24 bits (octets 2, 3,
and 4) can be divided into subnets and hosts as appropriate.
There are 126 Class A networks, with 16,777,214 (2
24
-2) possible hosts, for a total of
2,147,483,648 (2
31
) unique IP addresses. Class A networks make up 50% of the total available IP
addresses.
Class A Address Example:
Network Host
115 24.55.108
E.2.2 Class B Network Addresses
Class B network addresses are used for medium-sized networks that have 256 to 65534 hosts,
such as large college campuses. Every Class B network address has a 16-bit network prefix, with
the two highest order bits set to 1-0, followed by a 14-bit network number and a 16-bit host
number that identifies the host. IP addresses with a first octet from 128 to 191 are part of this
class. Class B networks are also referred to as “/16s” since they have a 16-bit network prefix.
There are 16,384 (2
14
) Class B networks, each with 65,534 (2
16
-2) possible hosts, for a total of
1,073,741,824 (2
30
) unique IP addresses. Class B networks make up 25% of the total available IP
addresses.
The Class B example in Figure E-1 has a major network address of 172.16. Sixteen bits (octets 3
and 4) are for local subnets and hosts.
Class B Address Example:
Network Host
145.24 52.108