ZyXEL Communications ES-2048 IP Address Classes and Network ID

ES-2048 User’s Guide

The following table shows the network number and host ID arrangement for classes A, B and C.

Table 83 Classes of IP Addresses

IP ADDRESS	OCTET 1	OCTET 2	OCTET 3	OCTET 4
Class A	Network number	Host ID	Host ID	Host ID
Class B	Network number	Network number	Host ID	Host ID
Class C	Network number	Network number	Network number	Host ID

An IP address with host IDs of all zeros is the IP address of the network (192.168.1.0 for example). An IP address with host IDs of all ones is the broadcast address for that network (192.168.1.255 for example). Therefore, to determine the total number of hosts allowed in a network, deduct two as shown next:

•A class C address (1 host octet: 8 host bits) can have 28 – 2, or 254 hosts.

•A class B address (2 host octets: 16 host bits) can have 216 – 2, or 65534 hosts.

A class A address (3 host octets: 24 host bits) can have 224 – 2 hosts, or approximately 16 million hosts.

IP Address Classes and Network ID

The value of the first octet of an IP address determines the class of an address.

•Class A addresses have a 0 in the leftmost bit.

•Class B addresses have a 1 in the leftmost bit and a 0 in the next leftmost bit.

•Class C addresses start with 1 1 0 in the first three leftmost bits.

•Class D addresses begin with 1 1 1 0. Class D addresses are used for multicasting, which is used to send information to groups of computers.

•There is also a class E. It is reserved for future use.

The following table shows the allowed ranges for the first octet of each class. This range determines the number of subnets you can have in a network.

Table 84 Allowed IP Address Range By Class

CLASS	ALLOWED RANGE OF FIRST OCTET (BINARY)	ALLOWED RANGE OF FIRST
		OCTET (DECIMAL)
Class A	00000000 to 01111111	0 to 127
Class B	10000000 to 10111111	128 to 191
Class C	11000000 to 11011111	192 to 223
Class D	11100000 to 11101111	224 to 239
Class E	11110000 to 11111111	240 to 255
(reserved)

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