Subnet Masks - ZyXEL Communications P-660R-F1 series User Manual

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
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 101 Allowed IP Address Range By Class
CLASS ALLOWED RANGE OF FIRST OCTET (BINARY)
Class A 00000000 to 01111111
Class B 10000000 to 10111111
Class C 11000000 to 11011111
Class D 11100000 to 11101111
Class E 11110000 to 11111111
(reserved)

Subnet Masks

A subnet mask is used to determine which bits are part of the network number, and which bits are part of the
host ID (using a logical AND operation).
A subnet mask has 32 bits. If a bit in the subnet mask is a "1" then the corresponding bit in the IP address is
part of the network number. If a bit in the subnet mask is "0" then the corresponding bit in the IP address is
part of the host ID.
240
8
– 2, or 254 hosts.
16
– 2, or 65534 hosts.
24
– 2 hosts, or approximately 16 million hosts.
ALLOWED RANGE OF FIRST
OCTET (DECIMAL)
0 to 127
128 to 191
192 to 223
224 to 239
240 to 255
P-660R-F1 Series User's Guide