A.4.5 HOSTS Files and the Domain Name Service (DNS)

Although you can certainly use IP addresses for communicating with other systems, this is undesirable for several reasons. IP addresses change quite often, so it is better to work with host names than to try and use fixed IP addresses. Furthermore, working with machine names like "WWW.UNOVERICA.COM" is much easier than trying to remember 32-bit numbers.

There are two common mechanism for matching names to IP addresses. One way is the use of a raw text file called "HOSTS", which simply maps device names to IP addresses. Each TCP/ IP system can have a local HOSTS file to use for this purpose. However, since each system must maintain their HOSTS file independently of the other systems, this method only works well in very small networks. It does not scale beyond a dozen or so systems, as the work required to add or update name-to-address mappings grows exponentially with every new mapping.

Another mechanism that is much more flexible and easier to manage in large environments is the Domain Name System (DNS). DNS servers manage hierarchical databases of IP addresses and their corresponding host names, and provide lookup services to DNS clients on the network. When a system needs to resolve a name-to-address mapping, it issues a DNS query to the local DNS server, which searches its database for the information and returns any addresses that are found, as shown in Figure A.17 below:

Figure A.17 When a client needs to locate a device by name, it queries the nearest DNS server, and receives an IP address in response.

If a devices' IP address changes for any reason, then the network administrator responsible for that network only has to update the DNS server's database, and all subsequent client queries are correctly resolved.

For more information on this subject, refer to Appendix C: Name Resolution Services. For more information on the IP protocol, refer to section A.4.1 Internet Protocol (IP) and Address Resolution Protocol (ARP). For more information on TCP and UDP, refer to section A.4.2 Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) and section A.4.3 IP Sockets and Ports. For more information on IP routing, refer to section A.4.4 Routing Information Protocol (RIP) and Open Shortest Path First (OSPF). For more information on TCP/ IP's mail protocols, refer to Appendix D: SMTP and POP3 Mail.

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