2.5 The Routing Table

Gateways route data between networks; but all network devices, hosts as well as gateways, must make routing decisions. For most hosts, the routing decisions are simple:

• If the destination host is on the local network, the data is delivered to the destination host.

• If the destination host is on a remote network, the data is forwarded to a local gateway.

Because routing is network-oriented, IP makes routing decisions based on the network portion of the address. The IP module determines the network part of the destination's IP address by applying the network mask to the address. If the destination network is the local network, the mask that is applied may be the local subnet mask. If no mask is provided with the address, the address class determines the network portion of the address.

After determining the destination network, the IP module looks up the network in the local routing table. [7] Packets are routed toward their destination as directed by the routing table. The routing table may be built by the system administrator or by routing protocols, but the end result is the same; IP routing decisions are simple table look-ups.

[7] This table is also called the forwarding table.

You can display the routing table's contents with the netstat -nr command. The -r option tells netstat to display the routing table, and the -n option tells netstat to display the table in numeric form. It's useful to display the routing table in numeric form because the destination of most routes is a network, and networks are usually referred to by network numbers.

On a Solaris system, the netstat command displays the routing table with the following fields:

Destination

The destination network (or host).

Gateway

The gateway to use to reach the specified destination.

Flags

The flags describe certain characteristics of this route. The possible flag values are:

U

Indicates that the route is up and operational.

H

Indicates this is a route to a specific host (most routes are to networks).

G

Means the route uses a gateway. The system's network interfaces provide routes to directly connected networks. All other routes use remote gateways. Directly connected networks do not have the G flag set; all other routes do.

D

Means that this route was added because of an ICMP Redirect Message. When a system learns of a route via an ICMP Redirect, it adds the route to its routing table, so that additional packets bound for that destination will not need to be redirected. The system uses the D flag to mark these routes.

Ref

The number of times the route has been referenced to establish a connection.

Use

The number of packets transmitted via this route.

Interface

The name of the network interface [8] used by this route.

[8] The network interface is the network access hardware and software that IP uses to communicate with the physical network. See Chapter 6, Configuring the Interface , for details.

The only two fields important for our current discussion are the destination and gateway fields. The following is a sample routing table:

% netstat -nr
Routing Table:
Destination  Gateway        Flags  Ref    Use   Interface
----------- -----------     -----  ----  -----  ---------
127.0.0.1   127.0.0.1       UH      1      298       lo0 
default     172.16.12.1     UG      2    50360         
172.16.12.0 172.16.12.2     U      40   111379       le0 
172.16.2.0  172.16.12.3     UG      4     1179         
172.16.1.0  172.16.12.3     UG     10     1113        
172.16.3.0  172.16.12.3     UG      2     1379        
172.16.4.0  172.16.12.3     UG      4     1119

The first table entry is the loopback route for the local host. This is the loopback address mentioned earlier as a reserved network number. Because every system uses the loopback route to send datagrams to itself, this entry is in every host's routing table. The H flag is set because it is a route to a specific host (127.0.0.1), not a route to an entire network (127.0.0.0). We'll see the loopback facility again when we discuss kernel configuration and the ifconfig command. For now, however, our real interest is in external routes.

Another unique entry in the routing table is the entry with the word "default" in the destination field. This entry is for the default route, and the gateway specified in this entry is the default gateway. The default route is the other reserved network number mentioned earlier: 0.0.0.0. The default gateway is used whenever there is no specific route in the table for a destination network address. For example, this routing table has no entry for network 192.168.16.0. If IP receives any datagrams addressed to this network, it will send the datagram via the default gateway 172.16.12.1.

You can tell from the sample routing table display that this host (peanut) is directly connected to network 172.16.12.0. The routing table entry for that network does not specify an external gateway; i.e., the routing table entry for 172.16.12.0 does not have the G flag set. Therefore, peanut must be directly connected to that network.

All of the gateways that appear in a routing table are on networks directly connected to the local system. In the sample shown above this means that, regardless of the destination address, the gateway addresses all begin with 172.16.12. This is the only network to which peanut is directly attached, and therefore it is the only network to which peanut can directly deliver data. The gateways that peanut uses to reach the rest of the Internet must be on peanut's subnet.

In Figure 2.5 the IP layer of each host and gateway on our imaginary network is replaced by a small piece of a routing table, showing destination networks and the gateways used to reach those destinations. When the source host (172.16.12.2) sends data to the destination host (172.16.1.2), it first determines that 172.16.1.2 is the local network's official address and applies the subnet mask. (Network 172.16.0.0 is subnetted using the mask 255.255.255.0.) After applying the subnet mask, IP knows that the destination's network address is 172.16.1.0. The routing table in the source host shows that data bound for 172.16.1.0 should be sent to gateway 172.16.12.3. Gateway 172.16.12.3 makes direct delivery through its 172.16.1.5 interface. Examining the routing tables shows that all systems list only gateways on networks they are directly connected to. Note that 172.16.12.1 is the default gateway for both 172.16.12.2 and 172.16.12.3. But because 172.16.1.2 cannot reach network 172.16.12.0 directly, it has a different default route.

Figure 2.5: Table-based routing

A routing table does not contain end-to-end routes. A route points only to the next gateway, called the next hop, along the path to the destination network. [9] The host relies on the local gateway to deliver the data, and the gateway relies on other gateways. As a datagram moves from one gateway to another, it should eventually reach one that is directly connected to its destination network. It is this last gateway that finally delivers the data to the destination host.

[9] As we'll see in Chapter 7, Configuring Routing , some routing protocols, such as OSPF and BGP, obtain end-to-end routing information. Nevertheless, the packet is still passed to the next-hop router.