The network layer attempts to deliver packets from a node on one network segment to another node that may be on another network segment. All network layer protocols use a header that includes both a source and destination address. Network layer addresses consist of two parts: A network segment prefix which identifies the network segment and a node suffix, which identifies the node on the segment.
An IP version 4 address is a total of 32 bits. The division of bits between the network segment prefix and the node varies. The network segment prefix is divided into 2 parts: The network number which is unique in the world and a subnet number with is assigned at the campus level.
An IP version 6 address is a total of 128 bits. The division between network segment prefix and node suffix will vary. The node suffix will be at least 48 bits to allow using an Ethernet address as the node suffix.
An IPX address is a total of 80 bits. 32 bits is used for the network number and 48 bits is used for the node number. In most cases the node number is the Ethernet address. Since we do not participate in a global IPX network, the network number is assigned at the campus level. The convention for IPX network numbers on the Madison campus is that the IPX network number is an IP address on the same network segment.
An Appletalk address is a total of 24 bits. 16 bits are used for the network number and 8 bits for the node number. The node number is chosen automatically at random from the node numbers that are not in use at the time the node starts up, so the node number may vary across startups.
The network layer does not guarantee delivery of packets. Packets may be dropped due to transmission errors, network congestion, an unknown address, or other reasons.