Domain Name System (DNS): One Mark Questions, Book Back Answers and Notes

About Domain Name System (DNS)

Medium ~90 min study

Every computer on the Internet is found by its IP address, a number. People are poor at remembering numbers but good at remembering names, so DNS was created to connect the two. When you type a website name, DNS looks up the matching IP address and hands it to your browser, much like a phone directory gives you a number when you know the person's name.

The unit builds up in order. First you learn what IPv4 and IPv6 addresses look like. Then you take a URL apart into protocol, host name, folder and file name. After that come the pieces of DNS itself: the tree-shaped name space, the name servers that store the records, the zones that divide responsibility, and the resolver on your own computer that starts every lookup.

Book-back MCQs from this unit often turn on exact numbers. Know that IPv4 has \(2^{32}\) addresses and IPv6 has \(2^{128}\), that a label can have at most 63 characters, that a full domain name can have at most 253 characters, and that the name space has 128 levels (0 to 127). Also be ready to tell an absolute URL from a relative one and to list, step by step, how a name is resolved.

What you'll learn

• Explain why DNS is needed and what it returns when given a domain name.
• Write an IPv4 address in dotted-decimal form and recognise an IPv6 address in colon-hexadecimal form.
• Split a URL into its protocol, host name, folder and file name.
• Tell an absolute URL from a relative URL and say where each is used.
• Describe the jobs of root, primary and secondary name servers.
• List in order the steps a resolver and name servers follow to find an IP address.

Before you start

• The client-server idea and basic network terms from the earlier networking units.
• Experience of typing web addresses into a browser.
• Binary numbers and powers of 2, so that \(2^{32}\) and \(2^{128}\) make sense.

Domain Name System (DNS) explained

How the Domain Name System Works

Why Names Are Mapped to Addresses

Computers route data using numbers, but a user would rather type a name. In the early network a single file listed every host name with its address. As the number of hosts grew, one file could not keep up, so DNS replaced it with a distributed database spread over many servers around the world. DNS is an application-layer service, and your browser uses it silently each time you visit a site.

IPv4 and IPv6 Addresses

An IP address identifies one device on a network. An IPv4 address is 32 bits long, so there are \(2^{32}\) possible addresses, about 4.3 billion. It is written in dotted-decimal form as four numbers from 0 to 255, for example 192.168.1.10. Because the world ran short of IPv4 addresses, IPv6 was introduced. It is 128 bits long, giving \(2^{128}\) addresses, and is written as eight groups of hexadecimal digits separated by colons, for example 2001:0000:3231:DFE1:0063:0000:0000:FEFB.

Parts of a URL

A URL (Uniform Resource Locator) is the full address of a file on the web. In http://www.example.com/docs/index.html the four parts are the protocol (http), the host or domain name (www.example.com), the folder (docs) and the file name (index.html). An absolute URL gives this complete address, starting from the protocol. A relative URL gives only the path from the current page's folder, such as docs/index.html, and is used for links within the same site.

The Name Space

To keep every name unique without one central controller, DNS arranges names as an inverted tree. The root sits at the top at level 0 and is written as a null (empty) label. The tree can go down 128 levels, numbered 0 to 127. Each node carries a label of at most 63 characters, and the labels read from a node up to the root, joined by dots, form its domain name. A domain is any subtree of this name space.

Name Servers

The DNS database is held by name servers. Root name servers are at the top; there are 13 sets of them, and they point queries towards the correct top-level domain servers such as those for .com or .in. A primary (master) name server keeps the original zone file, and changes are made there. A secondary (slave) name server keeps a read-only copy that it gets from the primary. It still answers queries, which spreads the load and keeps the domain reachable if the primary fails.

Zones, Resolvers and the Lookup

A zone is the part of the name space that one server or administrator is responsible for, stored as a zone file of resource records. The resolver is client software on your computer. When you type a name, the resolver asks the local name server. If that server has no cached answer, it asks a root server, which refers it to the top-level domain server, which in turn refers it to the authoritative server for the domain. That server returns the IP address, and the local server passes it back to your browser.

Common mistakes to avoid

• Treating domain names as case-sensitive. DNS ignores case, so WWW.GOOGLE.COM and www.google.com reach the same site.
• Mixing up absolute and relative URLs. A relative link breaks if the file is moved to a different folder; an absolute URL still works.
• Putting spaces or symbols like !, $ or & in a domain name. Only letters, digits and hyphens are allowed, and a name cannot start with a hyphen.
• Saying a secondary name server can edit records. Changes are made only on the primary; the secondary holds a read-only copy.
• Using zone and domain as the same word. A domain is a subtree of the name space, while a zone is only the part one server is responsible for, so a large domain can be split into several zones.

Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.

Frequently asked questions

What is the main difference between IPv4 and IPv6?

IPv4 addresses are 32 bits long and written in dotted decimal, which allows about 4.3 billion addresses. IPv6 addresses are 128 bits long and written in hexadecimal groups separated by colons. That is \(2^{128}\) addresses, enough for the huge growth in phones and other connected devices.

How does DNS find the IP address for a website name?

Your computer's resolver sends the name to the local name server. If the answer is not already cached, that server asks a root server, which refers it to the top-level domain server (for example .in). The TLD server refers it to the authoritative server for the domain, which returns the IP address. The local server then passes the address back to your browser.

Why do we need both primary and secondary name servers?

The primary name server holds the master zone file, and the administrator edits records there. The secondary copies that file and cannot change it. Because both answer queries, the load is shared, and the domain can still be resolved if the primary server goes down.

Which characters are allowed in a domain name?

Letters a to z, digits 0 to 9 and the hyphen. A hyphen cannot be the first character. Spaces and special symbols such as !, $ and & are not allowed. Each label can be up to 63 characters, and the full domain name can be up to 253 characters.

Is a DNS zone the same as a domain?

No. A domain is a whole subtree of the name space. A zone is the part of that tree one name server is responsible for. If a domain hands off some subdomains to other servers, the domain stays the same but its zone becomes smaller. One server can also manage more than one zone.

What happens if a DNS server fails?

If no name server can answer, your computer cannot turn names into IP addresses, so sites will not open by name even though they are still online. In practice this is rare, because secondary servers keep copies of the records and many servers cache recent answers.