A.4.1 Internet Protocol (IP) and Address Resolution Protocol (ARP)

IP, or the Internet Protocol, is the basic building block for all TCP/IP traffic, and works at the network-layer of the OSI reference model. It is responsible for tracking the network addresses of devices, and for determining how packets are delivered to devices on other networks.

Unlike IPX, which uses physical hardware addresses as the node address, IP uses manually- defined 32-bit addresses for each node on the global Internet. Part of the address defines the network that the node is on, and the rest of the address defines the node itself, as illustrated by Figure A.11 below. If the destination system is on a different network, then IP is used to send the packet to a router that can forward it on to the destination network.

Figure A.11 IP uses a two-part address consisting of a network address and workstation address.These addresses are manually assigned by the local system administrators, and must be unique across the entire Internet.

Packet delivery is handled at the physical layer, so IP must convert the Internet addresses into network addresses used by the local medium, which is typically the physical Ethernet addresses of the network adapters in use. IP uses the Address Resolution Protocol (ARP) to build this map in memory, as illustrated in Figure A.12 below. Since most local networks are fairly small, this map is not very big and only applies to devices on the local physical network.

Figure A.12 Because IP addresses do not correspond to hardware, the Addresss Resolution Protocol is used to determine who should receive which packets.

NOTE: IP does not guarantee delivery or provide error-correction services. These functions are provided by TCP.

For more information on the IP protocol and related concepts, refer to Appendix B: TCP/IP In Depth. For more information on TCP and UDP, refer to section A.4.2 Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) and section A.4.3 IP Sockets and Ports. For more information on IP routing, refer to section A.4.4 Routing Information Protocol (RIP) and Open Shortest Path First (OSPF). For more information on name resolution services, refer to section A.4.5 HOSTS Files and the Domain Name Service (DNS). For more information on TCP/ IP's mail protocols, refer to Appendix D: SMTP and POP3 Mail.

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