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A TCP/IP network usually interconnects a number of hosts. Your UnixWare 7 Release 7.2 host is connected to a TCP/IP network via a hardware network interface. Individual TCP/IP networks are in turn interconnected via IP routers. IP routers forward IP packets from one TCP/IP network to another and exchange routing information with each other to deliver packets across a number of networks. Other types of routers may forward traffic for protocol families other than TCP/IP.
If all of the hosts at your site are connected to a single TCP/IP network that is not interconnected with any other TCP/IP networks, an IP router is unnecessary. If your site comprises many TCP/IP networks, or if you want to interconnect your network with other TCP/IP networks or the Internet, you must configure the interconnections with IP routers so that all hosts can communicate. ``Planning a routing layout'' introduces the considerations that you should take into account when implementing routing on your networks.
Many types of machines may serve as IP routers. A number of vendors offer machines dedicated entirely to the function of IP routing. A system may act both as a host (offering network services such as remote login) and a router.
The IP routing mechanisms consist of:
If you use static routing, you maintain the routing table manually using the route(1Mtcp) command. Static routing is practical for a single network communicating with one or two other networks where the configuration rarely changes. However, as your network begins to communicate with more networks, the number of gateways increases as does the amount of time and effort required to maintain the routing table manually.
If you use dynamic routing, routing daemons running on the routers continuously exchange routing information and update the routing table automatically. Dynamic routing is suitable for complex multiply connected networks where the configuration may change frequently.
A router can build and maintain a dynamic routing table by running a routing daemon such as routed(1Mtcp) or gated(1Mtcp). A routing daemon manages the routing table by exchanging routing information with gateways and other routers. When routed runs on a router, it broadcasts its routing table and listens for broadcasts from other directly connected routers. It continually updates its routing table based on those broadcasts. A routing daemon that both broadcasts its routing tables and listens for broadcasts from other routers is termed ``active mode''. (In fact, it is now usual to use multicasting rather than broadcasting when exchanging routing information. However, this does not change the discussion of router configuration significantly.)
You can also create a mixture of static and dynamic routing on your networks. However, any static routes that you create are not advertised to other routers and are not updated by the routing daemons.
A routing daemon can also run on a host that itself is not a router. In this case, the daemon is configured to listen for broadcasts and update its local routing table; it does not broadcast to other machines. This is termed ``passive mode''.
When a machine can send an IP packet to another machine without going through a third machine, the route the packet will travel is said to be a ``direct route''. The selection of that route is called ``direct routing''. In ``Example internetwork'', the machine columbia can trace a direct route to the machines seine, thames and volga on the 172.16.1 network. columbia cannot reach rome, london or paris directly.
Example internetwork
When a machine wishing to send an IP packet to a second machine must send that packet through a third machine, the route the packet will travel is said to be an ``indirect route'' and the selection of the intermediary machine is called ``indirect routing''. The intermediary machine, a router, has connections to more than one network and is said to provide a gateway between the networks. Therefore, a router may also be called a ``network gateway''. In ``Example internetwork'', the machine volga is a network gateway between the 172.16.1 network and the 172.16.2 network. If the machine seine needs to send an IP packet to paris, it must send the packet to volga, which forwards the packet to paris.
The above example is a simple but common scenario, however, sometimes the source and destination hosts can be several networks, and thus routers, away from each other.