A. When host a wants to send to host b, if they are on the same local network then the IP protocol resolves the IP address to a physical address using ARP (Address Resolution Protocol), and the physical address (e.g. 00-05-f3-43-d3-3e) of the source and destination hosts are added to the IP datagram to form a frame, and using the frame, the two hosts can communicate directly with each other.
If the 2 hosts are not on the same local network, then they cannot communicate directly with each other, and instead have to go through a router. You have probably already come across a router when you install TCP/IP, as the default gateway is just a router that you have chosen to use as a means of communicating with hosts outside your local network if no specific route is known. A router can be a Windows NT computer with 2 or more network cards (one card for connection to each separate local network) or it can be a physical hardware device, such as Cisco routers.
Assuming our two hosts are not on the same local network, host A will check its routing table for a router that connects to the local network of host B. If it does not find a match then the data packets will be send to the "default gateway". In most cases, there will not be one router that connects straight to the intended recipient, rather the router will know of another route to pass on your packet, which will then goto another router etc.
For example:
Host A - 200.200.200.5
Host B - 200.200.199.6
Subnet Mask - 255.255.255.0
Router - 200.200.200.2 and 200.200.199.2
Host A's routing table - Network 200.200.199.0 use router 200.200.200.2
In this example, Host A would deduce that Host B is on a separate network, as its Network ID is 200.200.199. Host A would then check its routing table and see that it knows for network 200.200.199 (the zero means all) it should send to 200.200.200.2. The router would receive the packets and then forward them to network 200.200.199.
What actually happens is each router will have its own routing table that will point to other routes.
To actually configure a route, you use the route command, for example to configure a root for network 200.200.199 to use router 200.200.200.2 you would type
route -p add 200.200.199.0 mask 255.255.255.0 200.200.200.2
The -p makes the addition permanent, otherwise it will be lost with a reboot.
To view your existing information type route print.
