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20.2 THE TCP/IP PROTOCOL SUITE 645TABLE 20.2-1IP Address ClassesFirst OctetClass Network Address Host Address Decimal Value Range a Value of Initial BitsA 1st octet 2nd, 3rd, 4th octet 1–126 0B 1st, 2nd octet 3rd, 4th octet 128–191 10C 1st, 2nd, 3rd octet 4th octet 192–223 110a Some values are reserved and not available to users.[8,9]. With subnetting some of the leftmost bits of the host portion of the address (ina classful sense) are used to extend the network portion, freeing bits to increase thenumber of addressable networks. In supernetting the opposite is done, freeing bits toexpand the number of hosts in a network. Another effective scheme goes by thename of network address translation (NAT) [10]. With NAT, a packet networkuses public addresses from a small pool, assigning them to users when they needto access the Internet and returning them to the pool after use. Private addresses,assigned with no limitations, are used for intranetwork communication.Domain Names. A normal Internet user rarely sees IP addresses thanks to a userfriendlieraddressing scheme based on a hierarchical naming structure (domain namesystem—DNS [11,12]). DNS is based on domain and subdomain names, representedby alphanumeric strings, called labels. Labels are separated by dots, such as inwww.wiley.com. A worldwide network of hosts, called DNS servers, provides translationbetween domain names and IP addresses. A domain name is fully qualified(FQDN) if it contains all the labels necessary to identify a particular host on aglobal basis. For efficiency, non-fully-qualified names may be used locally, whenthere is no ambiguity.Fragmentation and Reassembly. This is the other basic function of IP, besidesaddressing. Different networks have different limits on packet size (maximum transmissionunit—MTU), depending on the characteristics of their data links. Sourcehosts have no knowledge of the MTUs of the networks traversed by packets andusually limit outgoing packets to a default value of 1500 bytes. When the IP logicin a router determines that the size of a received packet exceeds the MTU of the outgoinglink, it breaks the packet into smaller packets called fragments. The originalpacket is reassembled by the router where the packet exits the network with thesmaller MTU. A flag bit and the parameters Identification and Offset in the IPheader enable reassembly of the packet fragments.20.2.3 The Transmission Control Protocol (TCP)TCP is a transport layer protocol that uses the packet delivery services of IP toprovide host-to-host communication [13].