Figure 14 Direct and indirect deliveries

The IP Routing Table

A routing table is present on all IP nodes. The routing table stores information about IP networks and how they can be reached (either directly or indirectly). Since all IP nodes perform some form of IP routing, routing tables are not exclusive to IP routers. Any node loading the TCP/IP protocol will have a routing table. There are a series of default entries according to the configuration of the node and additional entries can be entered either manually through TCP/IP utilities or dynamically through interaction with routers.

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

1. The forwarding IP address:

For a direct delivery, the forwarding IP address is the destination IP address in the IP packet. For an indirect delivery, the forwarding IP address is the IP address of a router.

2. The interface to be used for the forwarding:

The interface identifies the physical or logical interface such as a network interface card that will be used to forward the packet to either its destination or the next router.

IP Routing Table Entry Types

An entry in the IP routing table contains the following information:

[Network ID, Subnet Mask, Next Hop, Interface, Metric]

• Network ID. The network ID corresponding to the route. The Network ID can be class-based, a subnet, a supernet, or an IP address for a host route. In the Windows NT IP routing table, this is the Network Address column.
• Subnet Mask. The subnet mask is used to match a destination IP address to the Network ID. In the Windows NT IP routing table, this is the Netmask column.
• Next Hop. The IP address of the next hop. In the Windows NT IP routing table, this is the Gateway Address column.
• interface. An indication of which network interface will be used to forward the IP packet.
• Metric. A number used to indicate the cost of the route so the best route among possible multiple routes to the same destination can be selected. A common use of the metric is to indicate the number of hops (routers crossed) to the Network ID. The route with the lowest metric is the best route.

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

• Directly Attached Network IDs. Routes for network IDs that are directly attached. For directly attached networks, the Next Hop field may be blank or contain the IP address of the interface on that network.
• Remote Network IDs. Routes for network IDs that are not directly attached but are available across other routers. For remote networks, the Next Hop field is the IP address of a local router in between the forwarding node and the remote network.
• 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 the IP address of the specified host and the subnet mask is 255.255.255.255.
• Default Route. The default route is designed to be used when a more specific Network ID or host route is not found. The default route network ID is 0.0.0.0 with the subnet mask of 0.0.0.0.

The Route Determination Process

To determine which the routing table entry will be used for the forwarding decision, the following process is used:

1. For each entry in routing table, perform a bit-wise logical AND between the Destination IP address and the Subnet Mask. Compare the result with the Network ID of the entry for a match.
2. The list of matching routes is compiled. The route that has the longest match (the route that matched the most amount of bits with the destination IP address) is chosen. The longest matching route is the most specific route to the destination IP address. If multiple entries with the longest match are found (multiple routes to the same network ID, for example), the router uses the lowest metric to select the best route. If multiple entries exist that are the longest match and the lowest metric, the router is free to choose which routing table entry to use.

The result of the route determination process is the choice of a single route in the routing table. The route chosen yields a forwarding IP address (the next hop IP address) and an interface (the port). If the route determination process fails to find a route, IP declares a routing error. For the sending host, an IP routing error is internally indicated to the upper layer protocol such as TCP or UDP. For a router, an ICMP Destination Unreachable-Network Unreachable message is sent to the source host.

Example Routing Table for Windows NT

Table 28 shows the default routing table for a Windows NT 4.0 host (not a router). The host has a single network interface card and has the IP address 157.55.27.90, subnet mask 255.255.240.0 (/20), and default gateway of 157.55.16.1.