Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 59
IP Address Restrictions

1.  Addresses cannot have the first four highest bits (in the first field) set to 1111. This is reserved for Class E networks only (a reserved network classification).

2.  The Class A address of 127.x is for a special function known as the loop–back function. It should never be visible on the network.

3.  The bits that define the host portion of the address cannot be set to all 1s or all 0s to indicate an individual address. These are reserved addresses. All 1s indicate a local subnet all hosts broadcast and all 0s indicate a network number.

4.  All 0s and all 1s are allowed in the subnet portion of an address as valid subnet addresses. Placing a 0 in the subnet is called subnet 0 (how clever) and most routers must be told that subnet 0 is supported. However, you must be careful when assigning all 1s to the subnet portion of the address. This is allowed (according to RFC 1812), but it can wreak havoc on those networks that use all subnets broadcast. If the subnet portion of the address is set to all 1s, this can be used as a directed broadcast. Routers will forward this type of datagram, if told to do so (they have to be configured).

5.  Any address with all 0s in the network portion of the address is meant to represent “this” network. For example, 0.0.0.120 is meant as host number 120 on “this” network (the network from which it originated).

6.  There is an old form of broadcasting known as the all–0s broadcast. This will take the form of 0.0.0.0. This form should not be used. 0.0.0.0 is used to indicate a default router (explained later).

7.  You can assign your own IP network numbers if you will never have access to the Internet or if you plan on using something like a Network Address Translator (NAT, RFC 1631). RFC 1918 allows three IP addresses to be used for private networks.

8.  Addresses cannot be out of the 255 (decimal) range for any of the 4 bytes. Therefore, an address of 128.6.200.655 is not a valid address. Likewise, an address of 420.6.7.900 is not a valid address assignment.