this packet is allowed to make before it is discarded by the net- work. Every time the packet enters a router, the router decrements the TTL value by one; when it reaches zero, the packet is discarded and the receiving device will ultimately invoke error control to ask for a resend. The Protocol field indicates the nature of the higher layer protocol that is carried within the packet. Encoded options include values for ICMP (1), TCP (6), UDP (17), or OSPF (89). The Header Checksum field is similar to a Frame-Check Sequence in HDLC and is used to ensure that the received IP header is free of errors. Keep in mind that IP is a connectionless protocol: This error check does not check the packet; it only checks the header. When transmitting packets it is always a good idea to have a source address and a destination address. You can figure out what they are for. Understanding IP Addresses As Figure 7-38 shows, IP addresses are 32-bits long. They are typically written as a sequence of four numbers, which represent the decimal value of each of the address bytes. These numbers are separated by periods ("dots" in telecom parlance), and the notation is referred to as dotted decimal notation. A typical address might be 168.152.20.10. These numbers are hierarchical; the hierarchy is described in the sec- tion that follows. IP addresses are divided into two subfields. The Network Identifier (NET_ID) subfield identifies the subnetwork that is connected to the Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Chapter 7 Figure 7-38 32-bit IP address: Each "segment" of the dotted decimal 192.168.1.1 address (192, 168, 1, 1) comprises eight bits. Internet. The NET ID is most commonly used for routing traffic between networks. On the other hand, the Host Identifier (HOST_ID) subfield identifies the address of a system (host) within a subnet