Table 270 Classes Of Ip Addresses; Table 271 Allowed Ip Address Range By Class - ZyXEL Communications ZyWall 5 Series User Manual

ZyWALL 5/35/70 Series User's Guide
The following table shows the network number and host ID arrangement for classes A, B and
C.

Table 270 Classes of IP Addresses

IP ADDRESS
Class A
Class B
Class C
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 2
• A class B address (2 host octets: 16 host bits) can have 2
A class A address (3 host octets: 24 host bits) can have 2
million hosts.
IP Address Classes and Network ID
The value of the first octet of an IP address determines the class of an IP address as already
stated. These are the details of how that range is determined.
• 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 271 Allowed IP Address Range By Class

CLASS
Class A
Class B
Class C
Class D
Class E
(reserved)
746
OCTET 1 OCTET 2
Network number Host ID
Network number Network number
Network number Network number
ALLOWED RANGE OF FIRST OCTET (BINARY)
00000000 to 01111111
10000000 to 10111111
11000000 to 11011111
11100000 to 11101111
11110000 to 11111111
OCTET 3 OCTET 4
Host ID Host ID
Host ID Host ID
Network number Host ID
8
– 2, or 254 hosts.
16
– 2, or 65534 hosts.
24
– 2 hosts, or approximately 16
ALLOWED RANGE OF FIRST
OCTET (DECIMAL)
0 to 127
128 to 191
192 to 223
224 to 239
240 to 255
Appendix E IP Addresses and Subnetting