D-Link DES-3326S manual Arp, 109

DES-3326S Layer 3 Fast Ethernet Switch User’s Guide

When IP is transmitting large files, large packets are much more efficient than small ones. It is preferable to use the largest possible packet size, but still be able to cross networks that require smaller packets.

To do this, IP can ‘negotiate’ packet size between the local and remote ends of a connection. When an IP connection is first made, the IPs at both ends of the connection state the largest packet they can handle. The smaller of the two is selected.

When a IP connection crosses multiple networks, it is possible that one of the intermediate networks has a smaller packet size limit than the local or remote network. IP is not able to determine the maximum packet size across all of the networks that may make up the route for a connection. IP has, therefore, a method to divide packets into multiple, smaller packets to cross such networks. This division of large packets into smaller packets is referred to as fragmentation.

A field in the TCP header indicates that a packet has been fragmented, and other information aids in the reassembly of the packets into the original data.

Gateways that connect networks of different packet size limits split the large packets into smaller ones and forward the smaller packets on their attached networks.

ARP

The Address Resolution Protocol (ARP) determines the MAC address and IP address correspondence for a network device.

A local computer will maintain an ARP cache which is a table of MAC addresses and the corresponding IP addresses. Before a connection with another computer is made, the local computer first checks its ARP cache to determine whether the remote

Switch Management and Operating Concepts 109