DNS – Domain Name System

DNS is a hierarchical representation of all the IT devices and their sub-domains of 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 becomes 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 level names and vice versa so that the user wanting to access a certain service in the network needs to remember the easily remember-able domain name than individual IP address. Software’s like Microsoft’s Active Directory, Unix/Linux based BIND and certain appliances with built-in DNS/DHCP servers can provide the DNS service in a network, to name a few.

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 devices dynamically by a DHCP server (which ought to be within a certain range and the range needs to be specified). The IP addresses assigned this way are not permanent – they are actually leased for a certain time to the devices. The next time, the same device may get the same IP address or a different one. One advantage of having a DHCP to assign IP addresses is, it will monitor and ensure that same IP address is not assigned twice in a network.  Normally, there is a range of addresses within which the DHCP server will assign to the nodes. But certain applications(and the devices that host them) that require static addresses can be excluded by specifying their static IP address to the DHCP server during the configuration. A default gateway (explained below) and the parent domain name of the company also need to be specified.

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 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, automatically. The IP addresses assigned to the clients can and does change 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 also used only within the company) and Public IP address, which are used in Internet for identifying websites or publicly connected servers etc. These public IP addresses are permanent and can be accessed from anywhere on the internet. There are addressing schemes which take care of the length, no. of bytes occupied by them etc. and the latest naming convention is IPv6 and you can read about it in detail, in 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 a subnet mask is to identify if the host is in the same network as the device. If it is, then the device sends a ARP request to determine the MAC address of the system to communicate with it in the data link layer. If the host is in a remote network, Then the network device routes the packet to a gateway in it’s routing table that is set to handle the network.

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

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