Internet host names are used because they are easier to remember than the long dotted decimal IP addresses. A host name is typically the name of a device that has a specific IP address and on the Internet is part of what is known as a Fully Qualified Domain Name (FQDN). An FQDN consists of a host name and a domain name. An example of an FQDN is hostname.company.com.
Although we have phone numbers and can remember them when we need to, life would be difficult if we had to remember the phone numbers of all our friends and associates. We might be able to remember the numbers of as many as 10 friends and relatives, but after that, things would get a bit difficult. Therefore, we remember their names and have directories of names and the phone numbers. Likewise, it’s easier to remember www.microsoft.com than it is to remember 184.108.40.206 and look up the name to associate the IP address with it.
The process of finding the IP address for any given host name is known as name resolution, and it can be performed in several ways: a HOSTS file, a request broadcast on the local network,DNS, and WINS. But before you read about that, you need to first understand Internet domainsand how they are organized.
Several automatic conversion systems are available to translate an IP address into a host name, and HOSTS is one of the simplest. You create a file called HOSTS, located in a particular folder or directory based on the requirements of the operating system, and enter a line in the file for every system. Here’s an example: 220.127.116.11 myserver.com #My server’s information 198.34.57.03 yourserver.com
Now comes the nasty part. You must store this ASCII file on every single workstation on your network; when you make a change, you must change the contents of the HOSTS file on every single workstation on your network. This is a simple but painful process inside a network. Butwhat happens if you want to go outside of this network to other networks or to the Internet?
The abbreviation DNS stands for Domain Name Service. You use DNS to translate host names and domain names to IP addresses, and vice versa, by means of a standardized lookup table that the network administrator defines and configures. The system works just like a giant telephone directory.
Suppose you are using your browser to surf the Web and you enter the URL http:// www.microsoft.com to go to the Microsoft home page. Your web browser then asks the TCP/IP protocol to ask the DNS server for the IP address of www.microsoft.com. When your web browser receives this address, it connects to the Microsoft web server and downloads the home page. DNS is an essential part of any TCP/IP network because it simplifies the task of remembering addresses; all you have to do is simply remember the host name and domain name.
A DNS zone is an administrative area or name space within a DNS domain. For example, sybex.com is a DNS domain, but there is a server that is authoritative over the sybex.com name space, or zone. An additional level could be added to sybex.com, making, for instance, networkbooks.sybex.com. The networkbooks zone would be handled by a particular server. The server holds the zone file, or DNS table, for that zone. DNS tables are composed of records. Most records are composed of a host name, a record type, and an IP address. There are several record types, including the address record, the mail exchange record, the CNAME record, and the SOA record.
There are primary DNS servers, which are authoritative for the zone for which they carry the zone file, and secondary DNS servers, which have a nonauthoritative copy of the zone file updated from the primary server. The DNS zone file must be changed only on the primary server that is authoritative for that zone. If changes are made to the secondary server, the changes will not be propagated elsewhere, and these changes will be lost during the next update from the primary server.
What this means is that whenever a change to any record is required, even one as simple as adding the IP address of a new server or changing the IP address of an established one, the change must be performed by the administrator, ISP, or other entity that hosts and has write access to the primary DNS server that is authoritative for the domain/zone in which the change occurs.
The address record, commonly known as the A record, maps a host name to an IP address. The following example shows the address record for a host called mail in the company.com domain:
mail.company.com. IN A 18.104.22.168
The mail exchange (MX) record points to the mail exchanger for a particular host. DNS is structured so that you can actually specify several mail exchangers for one host. This feature provides a higher probability that e-mail will actually arrive at its intended destination. The mail exchangers are listed in order in the record, with a priority code that indicates the order in which the mail exchangers should be accessed by other mail delivery systems.
If the first priority doesn’t respond in a given amount of time, the mail delivery system tries the second one, and so on. Here are some sample mail exchange records:
hostname.company.com. IN MX 10 mail.company.com.
hostname.company.com. IN MX 20 mail2.company.com.
hostname.company.com. IN MX 30 mail3.company.com.
In this example, if the first mail exchanger, mail.company.com, does not respond, the second one, mail2.company.com, is tried, and so on.
The CNAME record, or canonical name record, is also commonly known as the alias record and allows hosts to have more than one name. For example, your web server has the host namewww, and you want that machine to also have the name ftp so that users can use FTP to accessa different portion of the file system as an FTP root. You can accomplish this with a CNAMErecord. Given that you already have an address record established for the host name www, a CNAME record that adds ftp as a host name would look something like this:
www.company.com. IN A 22.214.171.124
ftp.company.com. IN CNAME www.company.com.
When you put all these record types together in a zone file, or DNS table, it might look like this:
mail.company.com. IN A 126.96.36.199
mail2.company.com. IN A 188.8.131.52
mail3.company.com. IN A 184.108.40.206
yourhost.company.com. IN MX 10 mail.company.com.
yourhost.company.com. IN MX 20 mail2.company.com.
yourhost.company.com. IN MX 30 mail3.company.com.
www.company.com. IN A 220.127.116.11
ftp.company.com. IN CNAME.
The SOA record, or start of authority record, contains global parameters for the zone and is easily the most crucial record in the zone file. There can be only one SOA per zone file. The SOA record might look something like this:
company.com. IN SOA ns.company.com. dns.support.company.com.
(55281 ; serial number
7200 ; refresh = 2h
3600 ; update retry = 1h
604800 ; expire = 7d
1800 ; minimum TTL = 30m)
WINS, or Windows Internet Naming Service, is an essential part of the Microsoft networking topology as long as NetBIOS is still in use. But before we get into the discussion of WINS, we must define a few terms, including these two protocols—NetBIOS and NetBEUI.
NetBIOS NetBIOS (pronounced “net-bye-ose”) is an acronym formed from network basic input/output system, a Session layer network protocol originally developed by IBM and Sytekto manage data exchange and network access. NetBIOS provides an application programming interface (API) with a consistent set of commands for requesting lower-level network services to transmit information from node to node, thus separating the applications from the under lying network operating system. Many vendors once provided either their own version of NetBIOSor an emulation of its communications services in their products.
NetBEUI NetBEUI (pronounced “net-boo-ee”) is an acronym formed from NetBIOS Extended User Interface, an implementation and extension of IBM’s NetBIOS transport protocol from Microsoft. NetBEUI communicates with the network through Microsoft’s Network Driver Interface Specification (NDIS). NetBEUI was once shipped with all versions of Microsoft’s operating systems and is generally considered to have a lot of overhead. It also has no structure to its addressing format, which does not allow determination of a network and therefore means it hasno routing capability, making it suitable only for small networks; you cannot build internetwork swith NetBEUI, and so it is often replaced with TCP/IP. Microsoft added extensions to NetBEUIin Windows NT to remove the limitation of 254 sessions per node. This extended version of Net-BEUI is called the NetBIOS Frame (NBF).
WINS is used in conjunction with TCP/IP and maps NetBIOS names to IP addresses. For example, you have a print server on your LAN that you have come to know as PrintServer1. In the past, to print to that server you needed only to remember its name and to select that name from a list. However, TCP/IP is a completely different protocol and doesn’t understand Net- BIOS names; it therefore has no way of knowing the location of those servers. That’s where WINS comes in.
Each time you access a network resource on a Windows network using TCP/IP, your system needs to know the host name or IP address. If there are no routers in your network, NetBIOSspeaking devices can simply broadcast their presence on the network and broadcast a request for the unknown MAC address of a known NetBIOS name, all without a WINS server. Once routers are introduced, however, the broadcasts that NetBIOS uses for resolution do not make it through the routers, so connectivity is lost to devices not on the same subnet as the requesting device. If WINS is installed in a routed environment, you can continue using the NetBIOS names that you have previously used to access the resources because WINS provides the cross-reference from NetBIOS name to IP address for you. Once the IP address is known, ARP can be used to obtain the MAC address after the packet has been routed to the proper IP subnet.
When you install and configure TCP/IP, as described later in this chapter, you’ll see a place to specify the WINS server addresses. These addresses are stored with the configuration, and TCP/IP uses them to query for host names and addresses when necessary. WINS is similar to DNS in that it cross-references host names to addresses; however, as mentioned earlier, WINS resolves NetBIOS names to IP addresses, but DNS resolves TCP/IP FQDNs to IP addresses.
Another major difference between WINS and DNS is that WINS builds its own reference tables dynamically but you have to configure DNS manually. Dynamic DNS (DDNS) does exist, but it is not yet implemented on the Internet. When a workstation running TCP/IP is booted and attached to the network, it uses the WINS address settings in the TCP/IP configuration to communicate with the WINS server. The workstation gives the WINS server various pieces of information about itself, such as the NetBIOS host name, the actual username logged on to the workstation, and the workstation’s IP address. WINS stores this information for use on the network and periodically refreshes it to maintain accuracy.
Microsoft, however, has developed a new DNS record—called DNS Server—that allows the DNS server to work in perfect harmony with a WINS server. The Microsoft DNS Server software was shipped with Windows NT and later server systems. Here’s how it works: The host name portion of the DNS FQDN can be looked up on the WINS server for hosts in the local domain. Thus, you need not build complex DNS tables to establish and onfigure name resolution on your server; Microsoft DNS relies entirely on WINS to tell it the addresses it needs to resolve. And because WINS builds its tables automatically, you don’t have to edit the DNS tables when addresses change; WINS takes care of this for you. This feature also is not available on the Internet.
You can use both WINS and DNS on your network, or you can use one without the other.
Your choice is determined by whether your network is connected to the Internet and whether your host addresses are dynamically assigned. When you are connected to the Internet, you must use DNS to resolve host names and addresses because TCP/IP depends on DNS service for address resolution. Addresses of both DNS and WINS servers can be supplied to a host with its Dynamic Host Configuration Protocol (DHCP) lease.
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