IP Addresses for Internal Networks


If you are designing an IP (IPv4) addressing scheme for your organization's internal network, you have some addressing flexibility that you would not have if you were designing a network that is part of the public Internet. Some special addresses have been made available for use in private networks.

The Internet Assigned Numbers Authority (IANA) in RFC 1597 (and more recently in RFC 1918) set aside specific network addresses for use in private networks in order to preserve the remaining pool of public addresses. These are:

  • - (1 Class A network address)
  • - (16 Class B network addresses)
  • - (255 Class C network addresses)

The above addresses are not used anywhere on the public Internet. You may use them for your organization's private internal network without registration or special permission. As long as your network remains isolated from the public Internet (and from other networks that use the same addresses), the RFC1918 addresses may be shared by many organizations. Internet connectivity can still be accomplished through the use of Network Address Translation (NAT) and other techniques.

There are several advantages to using the RFC 1918 addresses on your private network. An additional measure of security is provided for your network because TCP/IP data frames containing private addresses are not forwarded by backbone routers on the public Internet. Additional subnetting flexibility is available to you because you have available a much larger number of host and subnet addresses. You can, for example, dedicate an entire Class B address (or multiple Class C addresses) to your internal network.

Note: There are no more public IPv4 addresses available to hand out. You may be able to get a recycled IPv4 Class C network block - but you will probably need to justify every IP address in the block. To procure a publicly-registered Class A or B IPv4 address block, you would probably have to buy a company that has one. So, it only makes sense to maximize your use of these RFC 1918 IP addresses.

There are several things to consider when you develop your IP address plan. For example, it might be advantageous to consider using a block from - (Class B) rather than the typical 192.168.x.x (Class C) address block. The - addresses provide significantly greater address space, which you can use to address multiple VLANs, wide-area-network links, remote offices, etc. If you have many locations and VLANs, you might even consider using the - IP address block.

There are many benefits to designing your internal network using RFC 1918 IP addresses. Give FLG Networking Services a call to see how to make the best use of RFC1918 IP addresses on your network.