ZyXEL Communications P-660HW-T v2 - Manual (Page 298)

P-660HW-T v2 User’s Guide
298 Appendix F IP Subnetting
A class A address (3 host octets: 24 host bits) can have 2 24 – 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.
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.
Subnet masks are expressed in dotted decimal notation just like IP addresses. The “natural”
masks for class A, B and C IP addresses are as follows.
Table 131 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
(reserved) 11110000 to 11111111 240 to 255
Table 132 “Natural” Masks
CLASS NATURAL MASK
A 255.0.0.0