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Embed code for: 16.3 Dynamic Host Configuration Protocol
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16.3 Dynamic Host Configuration Protocol
Dynamic Host Configuration Protocol Section 16: Enabling Internet Connectivity Managing a network can be very timeconsuming. Network clients break, or are moved, and new clients are purchased that need network connectivity—these tasks are all part of the network administrator job. Depending on the number of IP hosts, manual configuration of IP addresses for every device on the network is virtually impossible. DHCP can greatly decrease the workload of the network administrator. DHCP automatically assigns an IP address from an IP address pool that is defined by the administrator. However, DHCP is much more than just a mechanism that allocates IP addresses. This service automates the assignment of IP addresses, subnet masks, gateways, and other IPrequired networking parameters. DHCP is built on a client/server model. The DHCP server hosts allocate network addresses and deliver configuration parameters to dynamically configured hosts. The term “client” refers to a host that is requesting initialization parameters from a DHCP server. A number of different devices can be DHCP clients, including Cisco IP phones, desktop PCs, laptops, printers, and even BluRay players. Just about any device that can be configured to participate on a TCP/IP network has the option of using DHCP to obtain its IP configuration. Depending on the actual DHCP server that is in use, there are three basic DHCP IP address allocation mechanisms: Dynamic allocation: Dynamic allocation of IP addresses is the most common type of address assignment. As devices boot and activate their Ethernet interfaces, the DHCP client service triggers a DHCP Discover broadcast that includes the MAC address of the DHCP client. If a DHCP server is listening on that IP subnet, it responds with a DHCP Offer message. As the name implies, the DHCP Offer message offers an unused IP address from the IP address pool that is on the DHCP server. If the IP address is acceptable, the DHCP client then sends a DHCP Request agreeing to the offered address. The DHCP server then marks the IP address as "in use" in its database and sends a final DHCP ACK to the DHCP client. The DHCP server also starts the countdown on a "lease timer." With a dynamic allocation, a DHCP client is given its IP configuration for a specified amount of time. When the lease time expires, the DHCP server can reclaim the address and return it to the address pool and lease it to another host. Automatic allocation: Automatic allocation of IP addresses is very similar to dynamic allocation, except the lease time is set to never expire. This setting results in the same DHCP client always being associated with the same IP address. Static allocation: Static allocation is an alternative that is generally used for devices such as servers and printers, where the device needs to remain at a given address more or less permanently. A static entry is made in the DHCP database that maps the MAC address to an IP address that is not part of the DHCP lease pool. The following examples show a simplified packet capture of a DHCP request. Source Destination Protocol Info 0.0.0.0 255.255.255.255 DHCP DHCP Discover 78:ac:c0:52:e8:bd ff:ff:ff:ff:ff:ff Source Destination Protocol Info 10.10.1.1 255.255.255.255 DHCP DHCP Offer 10.10.1.241 00:1b:d5:9c:34:27 ff:ff:ff:ff:ff:ff Source Destination Protocol Info 0.0.0.0 255.255.255.255 DHCP DHCP Request 78:ac:c0:52:e8:bd ff:ff:ff:ff:ff:ff Source Destination Protocol Info 10.10.1.1 255.255.255.255 DHCP DHCP ACK 00:1b:d5:9c:34:27 ff:ff:ff:ff:ff Up Next: Options for Configuring a Public IP Address