Dynamic Addressing and DHCP
DHCP allows a host to obtain an IP address quickly and dynamically. All that is required using DHCP is a defined range of IP addresses on a DHCP server. As hosts come online, they contact the DHCP server and request an address. The DHCP server chooses an address and allocates it to that host. With DHCP, the entire computers configuration can be obtained in one message (e.g. along with the IP address, the server can also send a subnet mask).
Layer 3 protocols determine whether data passes beyond the network layer to higher layers of the OSI model. A data packet must contain both a destination MAC address and a destination IP address. If it lacks one or the other, the data will not pass from Layer 3 to the upper layers. In this way, MAC addresses and IP addresses act as checks and balances for each other. After devices determine the IP addresses of the destination devices, they can add the destination MAC addresses to the data packets.
Devices can determine the MAC addresses they need to add to the encapsulated data in a variety of ways. Some keep tables that contain all the MAC addresses and IP addresses of other devices that are connected to the same LAN. They are called Address Resolution Protocol (ARP) tables, and they map IP addresses to the corresponding MAC addresses. ARP tables are sections of RAM memory, in which the cached memory is maintained automatically on each of the devices. It is a rare occasion when network administrator must make an ARP table entry manually. Each computer on a network maintains its own ARP table. Whenever a network device wants to send data across a network, it uses information provided by its ARP table.
When a source determines the IP address for a destination, the source consults its ARP table in order to locate the MAC address for the destination. If the source locates an entry in its table (destination IP address to destination MAC address), it binds, or associates, the IP address to the MAC address and uses it to encapsulate the data. The data packet is then sent out over the networking media to be picked up by the destination.
In order for a device to communicate with another device on another network, administrator must supply it with a default gateway. A default gateway is the IP address of the interface on the router that connects to the network segment on which the source host is located. The default gateways IP address must be in the same network segment as the source host.
If no default gateway is defined, communication is possible only on the devices own logical network segment. The computer that sends the data does a comparison between the IP address of the destination and its own ARP table. If it finds no match, it must have a default IP address to use. Without a default gateway, the source computer has no destination MAC address, and the message is undeliverable.
IP is a network layer protocol, and because of that, it can be routed over an internetwork, which is a network of networks. Protocols that provide support for the network layer are called routed or routable protocols.
The focus of this Domain is on the most commonly used routable protocol, which is IP. Even though it will concentrate on IP, it is important to know that there are other routable protocols.
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