ZyXEL Communications P-661H Series, User Manual, Page: 320 / 383

P-661H/HW Series User’s Guide
320 Appendix E
Since the first octet of a class “A” IP address must contain a “0”, the first octet of a class “A”
address can have a value of 0 to 127.
Similarly the first octet of a class “B” must begin with “10”, therefore the first octet of a class
“B” address has a valid range of 128 to 191. The first octet of a class “C” address begins with
“110”, and therefore has a range of 192 to 223.
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 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 as IP addresses are. The “natural”
masks for class A, B and C IP addresses are as follows.
Subnetting
With subnetting, the class arrangement of an IP address is ignored. For example, a class C
address no longer has to have 24 bits of network number and 8 bits of host ID. With
subnetting, some of the host ID bits are converted into network number bits. By convention,
subnet masks always consist of a continuous sequence of ones beginning from the left most bit
of the mask, followed by a continuous sequence of zeros, for a total number of 32 bits.
Table 129 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
Table 130 “Natural” Masks
CLASS NATURAL MASK
A 255.0.0.0
B 255.255.0.0
C 255.255.255.0