Running Linux, 5th Edition - Matthias Kalle Dalheimer [244]
IP acts as the mail room for the office building sending the letter. IP receives the envelope and wraps it in yet another envelope, with the IP address (office building address) of both the destination and the source affixed. The post office (which we haven't discussed quite yet) delivers the letter to the appropriate office building. There, the mail room unwraps the outer envelope and hands it to TCP/UDP, which delivers the letter to the appropriate office based on the port number (written on the inner envelope). Each envelope has a return address that IP and TCP/UDP use to reply to the letter.
To make the specification of machines on the Internet more humane, network hosts are often given a name as well as an IP address. The Domain Name System (DNS ) takes care of translating hostnames to IP addresses, and vice versa, as well as handling the distribution of the name-to-IP-address database across the entire Internet. Using hostnames also allows the IP address associated with a machine to change (e.g., if the machine is moved to a different network), without having to worry that others won't be able to "find" the machine once the address changes. The DNS record for the machine is simply updated with the new IP address, and all references to the machine, by name, will continue to work.
DNS is an enormous, worldwide distributed database. Each organization maintains a piece of the database, listing the machines in the organization. If you find yourself in the position of maintaining the list for your organization, you can get help from the Linux Network Administrator's Guide or TCP/IP Network Administration, both from O'Reilly. If those aren't enough, you can really get the full scoop from the book DNS and BIND (O'Reilly).
For the purposes of most administration, all you need to know is that either a daemon called named (pronounced "name-dee") has to run on your system, or you need to configure your system to use somebody else's name service—typically your ISP's or one running in your local network. This daemon or name service is your window onto DNS.
Now, we might ask ourselves how a packet gets from one machine (office building) to another. This is the actual job of IP, as well as a number of other protocols that aid IP in its task. Besides managing IP datagrams on each host (as the mail room), IP is also responsible for routing packets between hosts.
Before we can discuss how routing works, we must explain the model upon which TCP/IP networks are built. A network is just a set of machines that are connected through some physical network medium—such as Ethernet or serial lines. In TCP/IP terms, each network has its own methods for handling routing and packet transfer internally.
Networks are connected to each other via gateways (also known as routers ). A gateway is a host that has direct connections to two or more networks; the gateway can then exchange information between the networks and route packets from one network to another. For instance, a gateway might be a workstation with more than one Ethernet interface. Each interface is connected to a different network, and the operating system uses this connectivity to allow the machine to act as a gateway.
In order to make our discussion more concrete, let's introduce an imaginary network, made up of the machines eggplant, papaya, apricot, and zucchini. Figure 13-3 depicts the configuration of these machines on the network.
Figure 13-3. Network with two gateways
As you can see, papaya has two IP addresses—one on the 128.17.75 subnetwork and another on the 128.17.112 subnetwork. pear and pineapple are both on network 128.17.112, and pineapple is also on 128.17.30.
IP uses the network portion of the IP address to determine how to route packets between machines. To