IP addresses can be assigned to a device by a network administrator in one of two ways: statically, where the device's operating system is manually configured with the IP address, or dynamically, where the device's operating system is configured by using the DHCP.
When using DHCP, as soon as the device first connects to a network, it is automatically allocated an address by a DHCP server from a predefined pool. Some network devices, such as home broadband routers, provide a DHCP server service out of the box; otherwise, a DHCP server must be set up by a network administrator. DHCP is widely deployed, and it is particularly useful for networks where different devices may frequently connect and disconnect, such as public Wi-Fi hotspots or mobile networks.
DHCP environments require a DHCP server that's been configured with the appropriate parameters for the proposed network. The main DHCP parameters include the range or pool of available IP addresses, the correct subnet masks, and the gateway and server name addresses.
A DHCP server dynamically allocates IP addresses instead of having to depend on the static IP address and is responsible for assigning, leasing, reallocating, and renewing IP addresses. The protocol will assign an address that is available in a subnet or pool. This means that a new device can be added to a network without you having to manually assign it a unique IP address. DHCP can also combine static and dynamic IPs, and also determines how long an IP address is assigned to a device.
When a computer in a network wants to obtain a valid network configuration, usually when starting up the machine, it issues a DHCP Discover request. When this request—which is made through a UDP broadcast packet—reaches a DHCP server, a negotiation is established whereby the server grants the use of an IP, and other network parameters, to the client for a certain time.
It is important to take note of the following:
- The client does not need to have the network interface configured to issue a DHCP Discover request.
- The DHCP server can be on the same or a different subnet as the client will be on. If the client does not have network configuration, it cannot reach other subnets.
- When the DHCP server receives the DHCP request, Discover obtains the Mac address of the client, which may affect the IP address assigned to the client.
- The DHCP server grants network configuration to the client for a certain time. Before reaching the deadline, the client may try to renew the concession. If a concession occurs, the client must stop using the network configuration.
To make a DHCP request, you can use a client such as dhclient (native GNU/Linux) or the ipconfig/renew command (in the case of Windows). When a network configuration is obtained, the client uses it:
