Private IP addresses

Wed, 2007-07-18 08:37 by admin · Forum/category:

General

Before going into the details, if you are just looking for IP addresses for a small private network, either not connected to the Internet or connected to the Internet through a router with NAT (Network Address Translation), first try to set your IP address and DNS settings to automatic on all computers. This should work in most cases. If the router has a DHCP server, which most WAN (Wide Area Network) routers do, then the computers will obtain valid private IP addresses from the router. If there is no Internet connection, the computers will automatically assume APIPA (Automatic Private IP Address) addresses (169.254..., see below for details if you're interested).

If you need fixed IP addresses and the choice doesn't matter, use Class C addresses from only one of the following three ranges. These are most common in private networks.

192.168.0.1 - 192.168.0.254
192.168.1.1 - 192.168.1.254
192.168.2.1 - 192.168.2.254

If this is not enough, please read on.

Summary

This article is a brief summary of RFC-1918 (formerly RFC-1597): "Address Allocation for Private Internets". These are the recommended IP address ranges one can use for hosts that do not require direct access to the Internet. These addresses are filtered by Internet Routers and therefore do not have to be globally unique.

These addresses can be used with out fear of duplicating a unique IP address owned by another enterprise. If globally unique Internet addresses are required, contact your Internet Service Provider or the NIC (Network Information Center) at Hostmaster@NIC.DDN.MIL.

Detailed Information on Private Address Space

The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private networks. Note that the first block is nothing but a single class A network number, while the second block is a set of 16 contiguous class B network numbers, and third block is a set of 255 contiguous class C network numbers.

1 * Class A

Class A network IP address range = 10.0.0.0 - 10.255.255.255

For one Class A network:
Subnet mask = 255.0.0.0
Network address length = 8 bit
Computer address length = 24 bit

16 * Class B

Class B network IP address range = 172.16.0.0 - 172.16.255.255
Class B network IP address range = 172.17.0.0 - 172.17.255.255
Class B network IP address range = 172.18.0.0 - 172.18.255.255

Class B network IP address range = 172.31.0.0 - 172.31.255.255

For each of the 16 Class B networks:
Subnet mask = 255.255.0.0
Network address length = 16 bit
Computer address length = 16 bit

Alternatively, 16 * Class B combined

Combined Class B networks IP address range = 172.16.0.0 - 172.31.255.255

For all 16 Class B networks combined:
Subnet mask = 255.240.0.0
Network address length = 12 bit
Computer address length = 20 bit

256 * Class C

Class C network IP address range = 192.168.0.0 - 192.168.0.255
Class C network IP address range = 192.168.1.0 - 192.168.1.255
Class C network IP address range = 192.168.2.0 - 192.168.2.255

Class C network IP address range = 192.168.255.0 - 192.168.255.255

For each of the 256 Class C networks:
Subnet mask = 255.255.255.0
Network address = 24 bit
Computer address = 8 bit

Alternatively, 256 * Class C combined

Combined Class C networks IP address range = 192.168.0.0 - 192.168.255.255

For all 256 Class C networks combined:
Subnet mask = 255.255.0.0
Network address length = 16 bit
Computer address length = 16 bit

APIPA Addresses

Addition from outside the RFC: A link local Class B space is the APIPA (Automatic Private IP Address) range:

One Class B network IP address range = 169.254.0.0 - 169.254.255.255

For one Class B network:
Subnet mask = 255.255.0.0
Network address length = 16 bit
Computer address length = 16 bit

This range is used, for example, by Windows computers if their IP addresses are set to automatic, but no DHCP server is present.

The range is not a reserved private IP address space, but is often used as one. RFC-3927 says:

1.6. Alternate Use Prohibition

Note that addresses in the 169.254/16 prefix SHOULD NOT be configured manually or by a DHCP server. ...

See also this Microsoft Knowledge Base article:

How to use automatic TCP/IP addressing without a DHCP server

Related Additional Information

For a private domain name use

domain.com

or

experimental.com

Thus an example for a complete private computer host name could be:

mycomputer.domain.com

Authoritative sources can be found here:

Private, reserved, and special use address spaces – RFC-1918 (covers 10.0.0.0/8 172.16.0.0/12 192.168.0.0/16), also at: http://www.faqs.org/rfcs/rfc1918.html

Overview of all the special use and reserved IP address spaces – RFC 3330, also at: http://www.faqs.org/rfcs/rfc3330.html