Understanding DHCP

DHCP gives you centralized control over Internet Protocol (IP) addressing and more. If the network has a DHCP server, you can assign a dynamic IP address to any of the network interface cards (NICs) on a computer. Once DHCP is installed, you rely on the DHCP server to supply the basic information necessary for TCP/IP networking, which can include the following: IP address, subnet mask, and default gateway; primary and secondary Domain Name System (DNS) servers; primary and secondary Windows Internet Name Service (WINS) servers; and the DNS domain name .

The DHCP Client and the IP Address

A computer that uses dynamic addressing is called a DHCP client . When you boot a DHCP client, an IP address is retrieved from a pool of IP addresses defined for the network's DHCP server and assigned for a specified time period known as a lease . When the lease is approximately 50 percent expired , the client tries to renew it. If the client can't renew the lease, it'll try again before the lease expires . If this attempt fails, the client will try to contact a new DHCP server. IP addresses that aren't renewed are returned to the address pool. If the client is able to contact the DHCP server but the current IP address can't be reassigned, the DHCP server assigns a new IP address to the client.

The availability of a DHCP server doesn't affect startup or logon (in most cases). DHCP clients can start and users can log on to the local machine even if a DHCP server isn't available.

During startup, the client looks for a DHCP server. If a DHCP server is available, the client gets its configuration information from the server. If a DHCP server isn't available and the client's previous lease is still valid, the client pings the default gateway listed in the lease. A successful ping tells the client that it's probably on the same network it was on when it was issued the lease, and the client will continue to use the lease as described previously. A failed ping tells the client that it might be on a different network. In this case the client uses IP autoconfiguration. The client also uses IP autoconfiguration if a DHCP server isn't available and the previous lease has expired.

IP autoconfiguration works like this:

1. The client computer selects an IP address from the Microsoft-reserved class B subnet 169.254.0.0 and uses the subnet mask 255.255.0.0. Before using the IP address, the client performs an Address Resolution Protocol (ARP) test to make sure that no other client is using this IP address.

2. If the IP address is in use, the client repeats Step 1, testing up to 10 IP addresses before reporting failure.

   Note

   When a client is disconnected from the network, the ARP test always succeeds. As a result, the client uses the first IP address it selects.

   
   

3. If the IP address is available, the client configures the NIC with this address. The client then attempts to contact a DHCP server, sending out a broadcast every five minutes to the network. When the client successfully contacts a server, the client obtains a lease and reconfigures the network interface.

Checking IP Address Assignment

You can use Ipconfig to check the currently assigned IP address and other configuration information. To obtain information for all network adapters on the computer, type the command ipconfig /all at the command prompt. If the IP address has been assigned automatically, you'll see an entry for Autoconfiguration IP Address. In this example the autoconfiguration IP address is 169.254.98.59:

 Windows IP Configuration 

 Host Name .................: DELTA 

 Primary DNS Suffix ........: microsoft.com 

 Node Type .................: Hybrid 

 IP Routing Enabled.........: No 

 WINS Proxy Enabled.........: No 

 DNS Suffix Search List.....: microsoft.com 

 Ethernet adapter Local Area Connection: 

 Connection-specific DNS Suffix...: 

 Description ................: NDC ND5300 PnP Ethernet Adapter 

 Physical Address............: 05-82-C6-F8-FD-67 

 DHCP Enabled................: Yes 

 Autoconfiguration Enabled...: Yes 

 Autoconfiguration IP Address: 169.254.98.59 

 Subnet Mask ................: 255.255.0.0 

 Default Gateway ............: 

 DNS Servers ................: 

Understanding Scopes

Scopes are pools of IP addresses that you can assign to clients through leases and reservations . A reservation differs from a lease in that an IP address is assigned to a particular computer until you remove the reservation. This allows you to set semipermanent addresses for a limited number of DHCP clients.

You'll create scopes to specify IP address ranges that are available for DHCP clients. For example, you could assign the IP address range 192.168.12.2 “ 192.168.12.250 to a scope called Enterprise Primary. Scopes can use public or private IP addresses on

• Class A networks

  IP addresses from 1.0.0.0 to 126.255.255.255

• Class B networks

  IP addresses from 128.0.0.0 to 191.255.255.255

• Class C networks

  IP addresses from 192.0.0.0 to 223.255.255.255

• Class D networks

  IP addresses from 224.0.0.0 to 239.255.255.255

Note

The IP address 127.0.0.1 is used for local loopback.

A single DHCP server can manage multiple scopes. Three types of scopes are available:

• Normal scopes

  Used to assign IP address pools for class A, B, and C networks.

• Multicast scopes

  Used to assign IP address pools for class D networks. Computers use multicast IP addresses as secondary IP addresses in addition to a standard IP address assigned from a class A, B, or C network.

• Superscopes

  These are containers for other scopes and are used to simplify management of multiple scopes.

Tip

Although you can create scopes on multiple network segments, you'll usually want these segments to be in the same network class, such as all class C IP addresses. Don't forget that you must configure DHCP relays to relay DHCP broadcast requests between network segments. You can configure relay agents with the Routing and Remote Access Service (RRAS) and the DHCP Relay Agent Service. You can also configure some routers as relay agents .