ZyXEL Communications ZyXEL Vantage VSG-1000 - Manual (Page 120)

VSG-1000 Vantage Service Gateway
N IP Subnetting
Appendix E
IP Subnetting
IP Addressing
Routers “route” based on the network number. The router that delivers the data packet to the correct destination
host uses the host ID.
IP Classes
An IP address is made up of four octets (eight bits), written in dotted decimal notation, for example, 192.168.1.1. IP
addresses are categorized into different classes. The class of an address depends on the value of its first octet.
Class “A” addresses have a 0 in the left most bit. In a class “A” address the first octet is the network number
and the remaining three octets make up the host ID.
Class “B” addresses have a 1 in the left most bit and a 0 in the next left most bit. In a class “B” address the first
two octets make up the network number and the two remaining octets make up the host ID.
Class “C” addresses begin (starting from the left) with 1 1 0. In a class “C” address the first three octets make
up the network number and the last octet is the host ID.
Class “D” addresses begin with 1 1 1 0. Class “D” addresses are used for multicasting. (There is also a class
“E” address. It is reserved for future use.)
Chart 5 Classes of IP Addresses
IP ADDRESS: OCTET 1 OCTET 2 OCTET 3 OCTET 4
Class A 0 Network number Host ID Host ID Host ID
Class B 10 Network number Network number Host ID Host ID
Class C 110 Network number Network number Network number Host ID
Host IDs of all zeros or all ones are not allowed.
Therefore:
A class “C” network (8 host bits) can have 2 8 –2 or 254 hosts.
A class “B” address (16 host bits) can have 2 16 –2 or 65534 hosts.
A class “A” address (24 host bits) can have 2 24 –2 hosts (approximately 16 million hosts).
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.
Chart 6 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