So how does an IP packet addressed to a computer on the other side of the world find its way to its destination? The basic mechanism is very simple. On a LAN, every host can see every packet that is sent by every other host on that LAN. Normally, it will only do something with that packet if it is addressed to itself, or if the destination is a broadcast address.

A router is different. A router examines every packet, and compares the destination address to a table of addresses that it holds in memory. If it finds an exact match, it forwards the packet to an address associated with that entry in the table. This associated address may be the address of another network in a point- to- point link, or it may be the address of the next-hop router. If the router doesn't find a match, it runs through the table again, this time looking for a match on just the network ID part of the address. Again, if a match is found, the packet is sent to the address associated with that entry.

If a match is still not found, the router looks if a default next- hop address is present. If so, the packet is sent there. If no default address is present, the router sends an ICMP "host unreachable" or "network unreachable" message back to the sender. If you see this message, it usually indicates a router failure at some point in the network. The difficult part of a router's job is not how it routes packets, but how it builds up its table. In the simplest case, the router table is static: it is read in from a file at start- up. This is adequate for simple networks. You don't even need a dedicated piece of kit for this, because routing functionality is built into IP.

Dynamic routing is more complicated. A router builds up its table by broadcasting ICMP router solicitation messages, to which other routers respond. Routing protocols are used to discover the shortest path to a location. Routes are updated periodically in response to traffic conditions and availability of a route.

IP Routing Table contains information that describes a particular IP network route to a particular destination, under a particular policy. The results are shown as a table with the following columns

If.Index

Specifies the index of the interface for this route. This value identifies the local interface which the next hop of this route should be reached through.

Forward Destination IP

Specifies the IP address of the destination host. An entry with a value of 0.0.0.0 is considered as a default route.

Subnet Mask

Specifies the subnet mask of the destination host, indicates the mask to be logical-ANDed with the destination address before being compared to the value in the Forward Destination IP field.

Forward Policy

Specifies the set of conditions that would cause the selection of a multi-path route. IP TOS format is typically used. For more information, see RFC 1354.

Next Hop IP

Specifies the IP address of the next hop in the route. On remote routes, the address of the next system en route; Otherwise, 0.0.0.0.

Route Type

Specifies the route type. Possible values are:

Note that LOCAL refers to a route for which the next hop is the final, REMOTE refers to a route for which the next hop is not the final destination. Setting this object to the value INVALID has the effect of invalidating the corresponding entry in the ipForwardTable object. That is, it effectively disassociates the destination identified with said entry from the route identified with said entry. It is an implementation-specific matter as to whether the agent removes an invalidated entry from the table. Accordingly, management stations must be prepared to receive tabular information from agents that corresponds to entries not currently in use. Proper interpretation of such entries requires examination of the relevant ipForwardType object.

Forward Protocol

Specifies the protocol that generated the route - the routing mechanism via which this route was learned. Inclusion of values for gateway routing protocols is not intended to imply that hosts should support those protocols. Below is a list of possible protocols (The following routing protocols are associated with the IP transport).

The following are all dynamic routing protocols

Routes with a protocol identifier of LOCAL include:

• The loopback route
• The subnet route
• All networks broadcast route for subnetted interfaces
• All "1"s broadcast route
• Local multicast route
• Route to remote end of a PPP link

Age (sec)

Specifies the age of the route in seconds - the number of seconds since this route has been last updated or otherwise determined to be correct. Note that no semantics of 'too old' can be implied except through knowledge of the routing protocol by which the route was learned.

Hop Number

The Autonomous System Number of the Next Hop. When this is unknown or not relevant to the protocol indicated by Forward Protocol, then the hop number is zero.

Forward Metric

1 - Specifies a primary routing-protocol-specific metric value for this route. The semantics of this metric are determined by the routing-protocol. 2-5 (alternate) - an alternate routing metric for this route. The semantics of this metric are determined by the routing-protocol.

Connections | Statistics | IP-Address | IP-Routing | Adapters/Interface | ARP