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UNCLASSIFIED Background and Review Version 1.0g  UNCLASSIFIED 21   For any class, it is also possible to take the host portion of the address and further divide that range into two fields, which specify a subnet address and a host address respectively. This is done by specifying a parameter called a subnet mask. For a fuller discussion of subnetting see Albitton’s book [1] or one of the other references listed in Section 2.7.1. In addition to both source and destination addresses, there is a good bit of information in an IP header. It should be noted that the first 4 bits of an IP header contain a version number so new versions of the protocol can be implemented. Moreover the second 4 bits specify the length of the header. Thus it is quite feasible to introduce longer IP addresses.