26.1. What Is a Neighbor?

A host is your neighbor if it is connected to the same LAN (i.e., you are directly connected to it through either a shared medium or a point-to-point link) and it is configured on the same L3 network. For example, on an IP network, you can say that two hosts are neighbors if they are connected to the same LAN and each has at least one interface on the same IP subnet. Two such hosts can speak directly using the protocol associated with the medium that connects them (e.g., Ethernet). Another way to define a neighbor is to say that a host must be only one L3 hop away from its neighbor; its L3 routing table must provide a way for it to talk directly to the neighbor. Hosts that are not neighbors must communicate through a gateway or router.

Two hosts can still be neighbors if they are separated by a system on the L2 layer (a bridge). Part IV goes into more detail on this point, but we'll look at some simple examples here based on the IP networks of Figure 26-1.

Figure 26-1. Neighboring and non-neighboring hosts

Each topology in Figure 26-1 shows a different relationship between L3 and L2 addresses, which has implications for reaching neighbors:

Figure 26-1(a)

Host A and Host B belong to the same 10.0.1.0/24 IP subnet and therefore can talk directly, being just one L3 hop away from each other. They are neighbors.

Figure 26-1(b)

This shows a slightly more complex case. Host A and Host B still belong to the same subnet and can therefore talk directly to each other. Host A and Host C, on the other hand, belong to two different IP subnets; because of this they need to rely on a router (assuming they have been configured properly) to talk to each other. In this case, Host A and Host C can be considered two L3 hops away from each other.

Figure 26-1(c)

This shows a case of two hosts, connected to the same hub, that cannot talk to each other. Even if each host can receive whatever the other host transmits, they cannot talk to each other at the L3 layer because they have been configured with different IP subnets. Thus, Host A thinks it can reach only hosts within the subnet 10.0.1.0/24. It will not even try to send anything if the destination address is outside that subnet. This problem can be solved easily in numerous ways; we will see one in the following section, and more in other chapters.

Figure 26-1(d)

This shows a case where the subnet 10.0.1.0/24 actually consists of two LANs merged into one subnet through a hub or a bridge. We saw how they differ in Chapter 14, but from this chapter's perspective they can be considered equivalent. Note that the two interfaces used to merge the two LANs do not have IP addresses: this is because all three device types operate below the IP layer.

When two hosts are one L3 hop away from each other, they are usually one L2 hop away as well, as in Figure 26-1(a), (b), and (c). But this is not necessarily always the case, as shown in Figure 26-1(d), where Host A and router are one L3 hop apart (and therefore are neighbors) but two L2 hops apart.

Furthermore, the relationship between physical subnets (LANs) and logical subnets (i.e., IP subnets) is not always one-to-one, as shown in Figure 26-2(a). You can have multiple IP subnets on one LAN, or multiple LANs on one IP subnet. For example, Figure 26-1(c) shows two IP subnets on the same LAN, and Figure 26-1(d) shows two LANs connected by a hub on the same IP subnet (on the left side).While the former is not common, the latter is commonly used when configuring Proxy ARP or bridging. You can see an example of Proxy ARP configuration in the section "Final Common Processing" in Chapter 28, and examples of bridging in Part IV.

Figure 26-2(b) shows two groups of hosts configured to lie on different IP subnets. Even if the hosts of the two groups share the same LAN and are therefore able to talk to each other directly, they have to go through the router, which listens on both sides. The router could have two different Network Interface Cards, or NICs (as shown in Figure 26-2(b)), or a single NIC with multiple IP configurations. This scenario is pretty uncommon: it could be used, for example, to address a temporary shortage of equipment or a failure. For example, if you had the scenario in Figure 26-2(a) and LAN1 failed, you could move LAN1's hosts to LAN2 (including the Router's eth0 interface^[*]), and everything would work again without any need to change the IP subnet configuration of the hosts that were on LAN1. The hosts that were already on LAN2 will still access the other hosts through the router. Even if this scenario is uncommon, the kernel must be able to handle it properly. The implications of this scenario, especially when the router uses a single interface to access both subnets (i.e., eth0 is removed and its address is added to eth1), will be addressed in the section "Tunable ARP Options" in Chapter 28.

^[*] An alternative would be not to move the upper router's interface and simply add one IP address to the lower interface.

Figure 26-2. (a) IP_subnet-LAN 1:1; (b) IP_subnet-LAN n:1

In the rest of this chapter we will not explicitly mention the case of Figure 26-2(b), but you should keep in mind that setups like that one are possible and are not illegal.