Dynamic Host Configuration Protocol (DHCP)

Dynamic Host Configuration Protocol (DHCP) 

Configuring IP addressing on a large TCP/IP-based network can be a nightmare,especially if machines are moved from one network to another frequently. The Dynamic Host Configuration Protocol (DHCP) can help with the workload of configuring systems on a network by assigning addresses to systems on boot-up automatically.

     The process of dynamically assigning IP addresses is managed via a DHCP server.
The DHCP server is configured with a set of usable IP addresses, called a scope. The scope can also include the subnet mask, IP addresses of the default gateway, DNS servers, WINS servers, and other necessary addresses. When a PC comes online and is set up to use a DHCP server, it requests an IP address by transmitting a broadcast request packet looking for any DHCP servers on the network (known as DHCP Discovery). The DHCP server responds with an offer containing an IP address thatthe client can lease (known as the DHCP Offer). The client then accepts the offer by sending a request message for that address from the DHCP server (known as the DHCP Request), and then the server responds with an acknowledgment to the client that it has that address and additional settings for the lease time (known as the DHCP ACK). The DHCP server marks the IP address in its database as being in use so that it is not assigned again. When configuring the DHCP server, you will need to configure a scope with the following setting

• IP addresses The DHCP server issues an IP address to each DHCP client system on the network. Each system connected to a TCP/IP-based network is identified by a unique IP address. As you learned in this chapter, the IP address consists of four 8-bit octets separated by periods. The IP address is normally shown in dotted-decimal notation—for example, 192.10.24.62.
• Subnet mask The IP address actually consists of two parts: the network ID and the host ID. The subnet mask is used to identify the part of the IP address that is the network ID and the part that is the host ID. Subnet masks assign 1s to the network ID bits and 0s to the host ID bits of the IP address.
• Default gateway A default gateway is required when the client system
needs to communicate outside its own subnet. Normally, the default gateway is a router connected to the local subnet, which enables IP packets to be passed to other network segments.

Scope Options

A DHCP scope is the range of IP addresses and additional options that the DHCP server will hand out to the DHCP clients on the network. As previously mentioned, the IP address and subnet mask are required items that the DHCP scope must include. Another requirement in the scope is the lease duration. It specifies how long a DHCP client can use an IP address before it must renew it with the DHCP server. This duration can be set for an unlimited time period or for a predetermined time period. You have the option of configuring a scope to reserve a specific IP address for a DHCP client or even for a system on the network that is not DHCP enabled.

Servers

Several versions of Windows server products support having DHCP server capabilities, including Windows Server 2003 and Windows Server 2008. The main factor to consider if you have multiple subnets is that your routers must comply with RFC 1542 so that a DHCP server can receive the broadcast message from a client. It is wise to keep in mind that, if your DHCP server goes down and your DHCP clients cannot renew their lease, the clients will most likely not be able to access network resources.

      One of the benefits of using multiple DHCP servers is redundancy. Redundancy can prevent your network from going down. If you decide to use multiple DHCP servers, you should place them on different subnets to achieve a higher degree of fault tolerance in case one of the subnets becomes unavailable. You can manage multiple servers on different subnets with the DHCP Console, the graphical utility used to maintain and configure DHCP servers in Windows.

In most companies, two DHCP servers provide fault tolerance of IP addressing if one server fails or must be taken offline for maintenance. Each DHCP server has at least half of the available addresses in an active scope. The number of addresses on each DHCP server should be more than enough to provide addresses for all clients.

Supported Clients

The following operating systems can perform as DHCP clients on your network:

• Windows clients such as Windows 2000, XP, Vista
• Windows servers such as Windows 2000 Server, Windows Server 2003, and
Windows Server 2008
• Older Microsoft clients such as DOS (with network client software loaded), Windows 3.11, and Windows 9 x clients.
• Non-Microsoft operating systems such as Linux

Of course, DHCP clients are not limited to Microsoft operating systems. Any system that conforms to RFC 1541 can be a DHCP client. For example, you can have a UNIX, Linux, or Novell NetWare  client on the network that obtains an address from your DHCP server as well.

APIPA

Windows clients support a feature known as automatic private IP addressing (APIPA), which is a feature that provides that, when a Windows client boots up and cannot contact a DHCP server, it will configure itself automatically with a 169.254. x.y address. If there is something wrong with the DHCP server and all the systems on the network cannot obtain an address from the DHCP server, the clients will all assign themselves an address within the 169.254 address range and then be

able to communicate with one another.

       APIPA does not assign a default gateway, so you will be unable to access resources on a remote network and the Internet—but you can still communicate with systems on your network. When troubleshooting to find out why a machine cannot communicate on the network, watch for systems that have the 169.254.x.y addressrange because it means they could not find a DHCP server.

Boot Protocol

The Boot Protocol, known as BOOTP, is used by diskless workstations. When a diskless workstation boots, it does so using an EEPROM on the network card to allow it to load basic drivers and connect to the network by obtaining an IP address automatically.

    A BOOTP server, similar to a DHCP server, assigns the diskless workstation an address for the network to allow it to participate on the network. You will see the term BOOTP a lot when it comes to DHCP and routers; a BOOTP-enabled router will allow the DHCP broadcast to cross the router so that a DHCP server can be found on the other side of the network.