ZyXEL P-660HW-T1 v2 User Guide - Page 298

Subnet Masks

Page 298 highlights

P-660HW-T v2 User's Guide 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 131 Allowed IP Address Range By Class CLASS ALLOWED RANGE OF FIRST OCTET (BINARY) Class A Class B Class C Class D Class E (reserved) 00000000 to 01111111 10000000 to 10111111 11000000 to 11011111 11100000 to 11101111 11110000 to 11111111 ALLOWED RANGE OF FIRST OCTET (DECIMAL) 0 to 127 128 to 191 192 to 223 224 to 239 240 to 255 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 132 "Natural" Masks CLASS A NATURAL MASK 255.0.0.0 298 Appendix F IP Subnetting

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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
0
0000000 to
0
1111111
0 to 127
Class B
10
000000 to
10
111111
128 to 191
Class C
110
00000 to
110
11111
192 to 223
Class D
1110
0000 to
1110
1111
224 to 239
Class E
(reserved)
1111
0000 to
1111
1111
240 to 255
Table 132
“Natural” Masks
CLASS
NATURAL MASK
A
255.0.0.0