What are: DNS, DHCP, IP Addresses and Subnet Mask

This article gives an introduction to DNS and DHCP – what they are and why you need them. There is also a small explanation on what is an IP Address and what is a subnet mask. 

DNS – Domain Name System

DNS is a hierarchical representation of all the IT devices and their sub-domains in a (company) network. So, in case you see an address like mail.subdomain.company.com, you can easily recognize the name of the system (mail), name of the sub-domain (sub-domain), name of the company (company) and the name of the primary domain (.com).

This makes it easy to maintain a database of all the IT systems of an organization as each device is referred by a name and there is an appropriate sub-domain for locating them. It also becomes easier for various devices operating on the network to advertise and find each other’s location.

More importantly, a DNS Server converts the IP addresses in to domain names and vice versa so that the user wanting to access a certain service in the network can remember the (easily to remember) domain name than their individual IP address(es). Applications like Microsoft’s Windows 2008/2003, Unix/Linux based BIND and certain appliances with built-in DNS/DHCP servers can provide the DNS service in a network.

DHCP – Dynamic Host Control Protocol

A DHCP server automatically assigns the IP addresses for a device. Any device, before communicating with any other device in the network needs an IP address. These IP addresses are given to the network devices dynamically by a DHCP server (IP addresses ought to be within a certain range and the range  is already specified).

The IP addresses assigned this way are not permanent – they are actually leased for a certain amount of time to the devices (till they log-out of the network, for example). The next time when they log-in, the same device may get the same IP address or a different one. One advantage of having a DHCP Server to assign IP addresses is, it will monitor and ensure that same IP address is not assigned twice in a network.

Certain applications (and the devices that host them) require static addresses and these static IP addresses can be excluded from being allocated to other devices by the DHCP Server.

There are three more things that need to be explained while we are on the topic of DNS and DHCP servers. They are: IP addresses, Subnet mask and Default gateway.

IP addresses: IP addresses are the Network ID given to network devices to identify them on the network and to enable different devices to interact with each other. Unlike the physical addresses (MAC address) which is a permanent address set by the manufacturer, the IP addresses are either configured manually (static IP address) or they are configured by a DHCP server, where the IP addresses assigned to the clients changes every time the device boots up.

There are two type of IP addresses – Private IP address, which is a range of IP address that can be given to network devices of a company (which is  used only within the company) and Public IP address, which are used in Internet for identifying websites/ publicly connected servers etc. These public IP addresses can be obtained from an ISP and are permanent addresses which can be accessed from anywhere on the Internet.

There are addressing schemes (IPv4) which take care of the format of these IP addresses like their length, no. of bytes occupied by them,etc. The latest naming convention is IPv6, about which you can read in detail, from this link.

Subnet Mask: A subnet mask basically gives information on network and host portion of the address. It also helps to identify which part of IP address is reserved for the network and which part is available for host use. In short, it also enables to calculate if two IP addresses are in the same subnet or not. A network address (subnet) of a host can be determined by knowing the IP address and its subnet mask. The range of IP addresses possible on the network and the broadcast address of that network can also be identified.

One important application of the subnet mask is to identify if the host is in the same network as the device it wants to communicate with. If it is, then the host sends a ARP request to determine the MAC address of the system to communicate with it in the data link layer. If the device is in a remote network, the device routes the packet to a gateway in it’s routing table that is aware of the remote network.

A default gateway is a network setting on a computer that specifies the IP address of the computer / router the network traffic should be sent to, when the traffic is not on the same subnet as the sending computer.

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