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IP Addressing
15
Administration for Network Connectivity
555-233-504 — Issue 1 — April 2000 CID: 77730
1 Networking Overview
The IANA assigns a network address to an organization and a network administrator
in the organization assigns the Host IDs associated with that Network ID to nodes
within the organization’s network.
The following table shows the ranges of network and host IDs, and the total number
of IP addresses (# network IDs times # host IDs), for each class.
You can tell the class of an IP address by the first octet. For example, 191.221.30.101
is a Class B address and 192.221.30.101 is a Class C address.
Private IP Address Addresses on the Internet need to be unique to avoid ambiguity in message routing
over the Internet. To insure uniqueness, the Internet Assigned Number Authority
(IANA) controls the use of IP addresses. Organizations that maintain private
networks that never communicate with the Internet can use arbitrary IP addresses as
long as they are unique within the private network. To help prevent the duplication of
IP addresses on the Internet, the IANA has reserved the following ranges of IP
addresses for private networks:
1 Class A networks: 16.6 Million addresses: 10.0.0.0 --> 10.255.255.255
16 Class B networks: 1 Million addresses: 172.16.0.0 --> 172.31.255.255
256 Class C networks: 65,000 addresses:192.168.0.0 --> 192.168.255.255
These IP addresses can be used repeatedly in separate private networks, which are not
connected to the Internet. Routing tables prohibit the propagation of these addresses
over the Internet. (See RFC 1918). All other IP addresses are unique and must be
assigned by the IANA or ISP.
Network ID Range Host ID Range Total IP
Addresses
Class A 7 bits
126 Networks:
1 to 126
24 bits
16.8 Million Hosts per
network:
0.0.1 to 255.255.254
2.1 Billion
50%
Class B 14 bits,
16,382 Networks:
128.0 to 191.255
16 bits
65,534 Hosts per network
0.1 to 255.254
1.1 Billion
25%
Class C 21 bits,
2.1 Million Networks:
192.0.0 to 233.255.255
8 bits
254 Hosts per network:
1 to 254
0.5 Billion
12.5%
Classes
D&E
0.5 Billion
12.5%