Contents of an IP Routing Table

Contents of an IP Routing Table

The following are the fields of a typical IP routing table entry:

•	Destination The destination can be either an IP address or a class-based, subnetted, or supernetted network ID. In the Windows 2000 IP routing table, this column is named Network Destination.
•	Network Mask The bit mask that is used to match a destination IP address to the value in the Destination field. In the Windows 2000 IP routing table, this column is named Netmask.
•	Next-Hop The IP address to which the packet is forwarded. In the Windows 2000 IP routing table, this column is named Gateway.
•	Interface The network interface that is used to forward the IP packet.
•	Metric A number used to indicate the cost of the route so that the best route, among potentially multiple routes to the same destination, can be selected. A common use of the metric is to indicate the number of hops (the number of links or routers to cross) en route to the destination.

Routing table entries can be used to store the following types of routes:

•	Directly-attached network routes Routes for subnets to which the node is directly attached. For directly-attached network routes, the Next-Hop field can either be blank or contain the IP address of the interface on that subnet.
•	Remote network routes Routes for subnets that are available across routers and are not directly attached to the node. For remote network routes, the Next-Hop field is the IP address of a local router.
•	Host routes A route to a specific IP address. Host routes allow routing to occur on a per-IP address basis. For host routes, the network ID is a specific IP address and the network mask is 255.255.255.255.
•	Default route The default route is used when a more specific network or host route is not found. The default route destination is 0.0.0.0 with the network mask of 0.0.0.0. The next-hop address of the default route is typically the default gateway of the node.

Networking - Understanding the IP Routing Table

Understanding the IP Routing Table

All computers running any version of Windows and the supplied TCP/IP protocol use an IP routing table. The IP routing table stores information about destinations and how they can be reached. There are a series of default entries based on the configuration of the node. You can add entries with TCP/IP utilities or entries can be added dynamically through interaction with routers.

When an IP packet is forwarded, the IP routing table is used to determine:

1.	The next-hop IP address For a direct delivery (the destination is a neighboring node), the next-hop IP address is the destination address in the packet. For an indirect delivery (the destination is not a neighboring node), the next-hop address is the address of a router.
2.	The next-hop interface The next-hop interface identifies either a physical interface (for example, a network adapter) or a logical interface (for example, a tunneling interface) that is used to forward the packet.

After the next-hop address and interface are determined, the packet is passed to the Address Resolution Protocol (ARP). For LAN technologies such as Ethernet and Token Ring, ARP attempts to resolve the link-layer address (also known as the media access control [MAC] address) for the next-hop address, and forward the packet by using the next-hop interface.

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