Default Gateway - Example1

Example1

An office network consists of six hosts and a router is given as:

Hosts addresses:

• 192.168.4.2
• 192.168.4.4
• 192.168.4.5
• 192.168.4.6
• 192.168.4.7
• 192.168.4.8

Router (this side) address:

• 192.168.4.1

The network has a subnet mask of:

• 255.255.255.0 (/24 in CIDR notation)

Thus the usable network ranges from addresses 192.168.4.1 to 192.168.4.254. (TCP/IP defines the addresses 192.168.4.0 and 192.168.4.255 for special functions.)

The office's hosts will send packets addressed to IPs within this range directly, by resolving the destination IP address into a MAC address through an ARP sequence (if not already known through the host's ARP cache) and then enveloping the IP packet into a layer 2 (MAC) packet addressed to the destination host.

Packets addressed outside of this range (for this example, a packet addressed to 192.168.12.3) cannot travel directly to the destination. Instead they must be sent to the default gateway for further routing to their ultimate destination. In this example, the default gateway uses the IP address 192.168.4.1, which is resolved into a MAC address with ARP in the usual way. Note that the destination IP address remains 192.168.12.3, but the next-hop physical address is that of the gateway, rather than of the ultimate destination.