Every computer that runs TCP/IP makes routing decisions. These decisions are controlled by the IP routing table. To display the IP routing table on computers running
The following table shows an example of an IP routing table. This example is for a computer running
Description | Network destination | Netmask | Gateway | Interface | Metric |
---|---|---|---|---|---|
Default route | 0.0.0.0 | 0.0.0.0 | 10.0.0.1 | 10.0.0.169 | 30 |
Loopback network | 127.0.0.0 | 255.0.0.0 | 127.0.0.1 | 127.0.0.1 | 1 |
Local network | 10.0.0.0 | 255.0.0.0 | 10.0.0.169 | 10.0.0.169 | 30 |
Local IP address | 10.0.0.169 | 255.255.255.255 | 127.0.0.1 | 127.0.0.1 | 30 |
Multicast addresses | 224.0.0.0 | 240.0.0.0 | 10.0.0.169 | 10.0.0.169 | 30 |
Limited broadcast address | 255.255.255.255 | 255.255.255.255 | 10.0.0.169 | 10.0.0.169 | 1 |
Note
The routing table is built automatically, based on the current TCP/IP configuration of your computer. Each route occupies a single line in the displayed table. Your computer searches the routing table for an entry that most closely matches the destination IP address.
Your computer uses the default route if no other host or network route matches the destination address included in an IP datagram. The default route typically forwards an IP datagram (for which there is no matching or explicit local route) to a default gateway address for a router on the local subnet. In the previous example, the default route forwards the datagram to a router with a gateway address of 10.0.0.1.
Because the router that corresponds to the default gateway contains information about the network IDs of the other IP subnets within the larger TCP/IP internet, it forwards the datagram to other routers until the datagram is eventually delivered to an IP router that is connected to the specified destination host or subnet within the larger network.
The following sections describe each of the columns displayed in the IP routing table: network destination, netmask, gateway, interface, and metric.
The network destination is used with the netmask to match the destination IP address. The network destination can range from 0.0.0.0 for the default route through 255.255.255.255 for the limited broadcast, which is a special broadcast address to all hosts on the same network segment.
The netmask is the subnet mask that is applied to the destination IP address when matching it to the value in the network destination. When netmask is written in binary, a "1" must match and a "0" need not match. For example, a default route uses a 0.0.0.0 netmask that translates to the binary value 0.0.0.0, so bits need not match. A host route—a route that matches an IP address—uses a 255.255.255.255 netmask that translates to the binary value 11111111.11111111.11111111.11111111, so all of the bits must match.
The gateway address is the IP address that the local host uses to forward IP datagrams to other IP networks. This is either the IP address of a local network adapter or the IP address of an IP router (such as a default gateway router) on the local network segment.
The interface is the IP address that is configured on the local computer for the local network adapter that is used when an IP datagram is forwarded on the network.
A metric indicates the cost of using a route, which is typically the number of hops to the IP destination. Anything on the local subnet is one hop, and each router crossed after that is an additional hop. If there are multiple routes to the same destination with different metrics, the route with the lowest metric is selected.
For information about adding routes to the IP routing table, see To add a static IP route. For information about deleting routes in the IP routing table, see To remove a static IP route.
The following shows the default routing table for a multihomed
Adapter | Description | Network destination | Netmask | Gateway | Interface | Metric |
---|---|---|---|---|---|---|
1 | Default route | 0.0.0.0 | 0.0.0.0 | 10.0.0.1 | 10.0.0.169 | 20 |
2 | Default route | 0.0.0.0 | 0.0.0.0 | 192.168.0.1 | 192.168.0.200 | 30 |
1 | Loopback network | 127.0.0.0 | 255.0.0.0 | 127.0.0.1 | 127.0.0.1 | 1 |
1 | Local network | 10.0.0.0 | 255.0.0.0 | 10.0.0.169 | 10.0.0.169 | 20 |
1 | Local IP address | 10.0.0.169 | 255.255.255.255 | 127.0.0.1 | 127.0.0.1 | 20 |
2 | Local network | 192.168.0.0 | 255.255.0.0 | 192.168.0.200 | 192.168.0.200 | 30 |
2 | Local IP address | 192.168.0.200 | 255.255.255.255 | 127.0.0.1 | 127.0.0.1 | 30 |
2 | Subnet broadcast | 192.168.0.255 | 255.255.255.255 | 192.168.0.200 | 192.168.0.200 | 30 |
1 | Multicast address | 224.0.0.0 | 240.0.0.0 | 10.0.0.169 | 10.0.0.169 | 20 |
2 | Multicast address | 224.0.0.0 | 240.0.0.0 | 192.168.0.200 | 192.168.0.200 | 30 |
1 | Limited broadcast | 255.255.255.255 | 255.255.255.255 | 10.0.0.169 | 10.0.0.169 | 1 |
2 | Limited broadcast | 255.255.255.255 | 255.255.255.255 | 192.168.0.200 | 192.168.0.200 | 1 |
Note
For information about enabling IP forwarding on a multihomed computer running a
Notes