77
IP Network Classes
Class
Maximum Number
of Networks in
Class
Network Addresses (Host
Portion in Parenthesis)
Maximum Number of
Hosts per Network
A
126
1(.0.0.0) to 126(.0.0.0)
16,777,214
B
16,382
128.1(.0.0) to 191.254(.0.0)
65,534
C
2,097,150
192.0.1(.0) to 223.255.254(.0)
254
Note:
All network addresses outside of these ranges (Class D and E) are either reserved or set aside for
experimental networks or multicasting.
When an IP address's host portion contains only zero(s), the address identifies a network and not a host. No
physical device may be given such an address.
The network portion must start with a value from 1 to 126 or from 128 to 223. Any other value(s) in the network
portion may be from 0 to 255, except that in class B the network addresses 128.0.0.0 and 191.255.0.0 are
reserved, and in class C the network addresses 192.0.0.0 and 223.255.255.0 are reserved.
The value(s) in the host portion of a physical device's IP address can be in the range of 0 through 255 as long as
this portion is not all-0 or all-255. Values outside the range of 0 to 255 can never appear in an IP address (0 to
255 is the full range of integer values that can be expressed with eight bits).
The network portion must be the same for all the IP devices on a discrete physical network (a single Ethernet
LAN, for example, or a WAN link). The host portion must be different for each IP device
—
or, to be more
precise, each IP-capable port or interface
—
connected directly to that network.
The network portion of an IP address will be referred to in this manual as a
network number
; the host portion
will be referred to as a
host number
.
To connect to the Internet or to any private IP network that uses an Internet-assigned network number, you must
obtain a registered IP network number from an Internet-authorized network information center. In many
countries you must apply through a government agency, however they can usually be obtained from your
Internet Service Provider (ISP).
If your organization's networks are, and will always remain, a closed system with no connection to the Internet
or to any other IP network, you can choose your own network numbers as long as they conform to the above
rules.
If your networks are isolated from the Internet, e.g. only between your two branch offices, you can assign any IP
Addresses to hosts without problems. However, the Internet Assigned Numbers Authority (IANA) has reserved
the following three blocks of IP Addresses specifically for private (stub) networks:
Class
Beginning Address
Ending Address
A
10.0.0.0
10.255.255.255
B
172.16.0.0
172.31.255.255
C
192.168.0.0
192.168.255.255
It is recommended that you choose private network IP Addresses from the above list. For more information on
address assignment, refer to RFC 1597,
Address Allocation for Private Internets
and RFC 1466,
Guidelines for
Management of IP Address Space
.
Subnet Mask
In the absence of subnetworks, standard TCP/IP addressing may be used by specifying subnet masks as shown
below.
