DHCP
Q. What is DHCP?
A. DHCP stands for "Dynamic Host Configuration Protocol". DHCP (Dynamic Host Configuration Protocol) is a communications protocol that lets network administrators centrally manage and automate the assignment of Internet Protocol (IP) addresses in an organization's network.
DHCP, the Dynamic Host Configuration Protocol, describes the means by which a system can connect to a network and obtain the necessary information for communication upon that network.
Q. Who Created It? How Was It Created?
A. DHCP was created by the Dynamic Host Configuration Working Group of the Internet Engineering Task Force (IETF; a volunteer organization which defines protocols for use on the Internet). As such, its definition is recorded in an Internet RFC and the Internet Activities Board (IAB) is asserting its status as to Internet Standardization. As of this writing (June 1998), DHCP is an Internet Draft Standard Protocol and is Elective. BOOTP is an Internet Draft Standard Protocol and is recommended.
Q. How DHCP Works?
A. DHCP uses a client-server model. The network administrator establishes one or more DHCP servers that maintain TCP/IP configuration information and provide it to clients. The server database includes the following:
· Valid configuration parameters for all clients on the network.
· Valid IP addresses maintained in a pool for assignment to clients, plus reserved addresses for manual assignment.
· Duration of a lease offered by the server. The lease defines the length of time for which the assigned IP address can be used.
With a DHCP server installed and configured on your network, DHCP-enabled clients can obtain their IP address and related configuration parameters dynamically each time they start and join the network. DHCP servers provide this configuration in the form of an address-lease offer to requesting clients.
A. With a DHCP server installed and configured on your network, DHCP-enabled clients can obtain their IP address and related configuration parameters dynamically each time they start and join the network. DHCP servers provide this configuration in the form of an address-lease offer to requesting clients.
1. The DHCP client requests an IP address by broadcasting a DHCP Discover message to the local subnet.
2. The client is offered an address when a DHCP server responds with a DHCP Offer message containing IP address and configuration information for lease to the client. If no DHCP server responds to the client request, the client can proceed in two ways:
· If it is a Windows 2000–based client, and IP auto-configuration has not been disabled, the client self-configures an IP address for its interface.
· If the client is not a Windows 2000–based client, or IP auto-configuration has been disabled, the client network initialization fails. The client continues to resend DHCP Discover messages in the background (four times, every 5 minutes) until it receives a DHCP Offer message from a DHCP server.
3. The client indicates acceptance of the offer by selecting the offered address and replying to the server with a DHCP Request message.
4. The client is assigned the address and the DHCP server sends a DHCH ACK message, approving the lease. Other DHCP option information might be included in the message.
5. Once the client receives acknowledgment, it configures its TCP/IP properties using any DHCP option information in the reply, and joins the network.
In rare cases, a DHCP server might return a negative acknowledgment to the client. This can happen if a client requests an invalid or duplicate address. If a client receives a negative acknowledgment (DHCP Nack), the client must begin the entire lease process again.
Q. At what layer of OSI it functions?
A. DHCP works at Data link Layer. (Layer 2)
Q. What is DORA?
A. Finally, the chosen DHCP server sends the lease information (the IP address, potentially a subnet mask, DNS server, WINS server, WINS node type, domain name, and default gateway) to the workstation in a message called the DHCP ACK (data communications jargon for acknowledge). You can remember the four parts of a DHCP message by the mnemonic DORA - Discover, Offer, Request, and ACK.
Q. What is the default Lease Period in DHCP Client/Server communication?
A. The default lease is 8 days, after which a computer has to renew their use of the address they've been leased by your DHCP server.
There are certain situations however when you might want to lengthen this lease period to several weeks or months or even longer. These situations include (a) when you have a stable network where computers neither join or are removed or relocated; (b) when you have a large pool of available IP addresses to lease from; or (c) when your network is almost saturated with very little available bandwidth and you want to reduce DHCP traffic to increase available bandwidth (not by much, but sometimes every little bit helps).
Q. How does DHCP lease works?
A. The first time a DHCP-enabled client starts and attempts to join the network; it automatically follows an initialization process to obtain a lease from a DHCP server. Figure 4.2 shows the lease process.
Figure 4.2 DHCP Lease Process
3. The DHCP client requests an IP address by broadcasting a DHCP Discover message to the local subnet.
4. The client is offered an address when a DHCP server responds with a DHCP Offer message containing IP address and configuration information for lease to the client. If no DHCP server responds to the client request, the client can proceed in two ways:
· If it is a Windows 2000–based client, and IP auto-configuration has not been disabled, the client self-configures an IP address for its interface.
· If the client is not a Windows 2000–based client, or IP auto-configuration has been disabled, the client network initialization fails. The client continues to resend DHCP Discover messages in the background (four times, every 5 minutes) until it receives a DHCP Offer message from a DHCP server.
6. The client indicates acceptance of the offer by selecting the offered address and replying to the server with a DHCP Request message.
7. The client is assigned the address and the DHCP server sends a DHCH ACK message, approving the lease. Other DHCP option information might be included in the message.
8. Once the client receives acknowledgment, it configures its TCP/IP properties using any DHCP option information in the reply, and joins the network.
In rare cases, a DHCP server might return a negative acknowledgment to the client. This can happen if a client requests an invalid or duplicate address. If a client receives a negative acknowledgment (DHCP Nack), the client must begin the entire lease process again.
Q. How can you backup configuration file of DHCP server?
A. DHCP database backs itself up automatically every 60 minutes to the %SystemRoot%\System32\Dhcp\Backup\Jet directory. This interval can be changed:
- Start the registry editor
2. Move to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DHCPServer\Parameters\BackupInterval
- Double click on BackupInterval and set to the number of minutes you want the backup to be performed. Click OK
- Close the registry editor
- Stop and restart the DHCP server service (Start - Settings - Control Panel - Services - DHCP Server - Start and Stop)
You could backup the %SystemRoot%\System32\Dhcp\Backup\Jet directory if you wish.
Q. Had you maintained/created any technical reference documentation on DHCP Server/Client?
A. Yes.
Q. What is TCP/IP port no. used for DHCP service?
A. DHCP uses the same two IANA assigned ports as BOOTP: 67/udp for the server side, and 68/udp for the client side.
Q. What is VLAN?
A. A virtual LAN, commonly known as a vLAN or as a VLAN, is a method of creating independent logical networks within a physical network.
A VLAN consists of a network of computers that behave as if connected to the same wire - even though they may actually be physically connected to different segments of a LAN. Network administrators configure VLANs through software rather than hardware, which make them extremely flexible.
A. DHCP and VLANs, which are very different in concept, are sometimes cited as different solutions to the same problem. While they have a goal in common (easing moves of networked computers), VLANs represent a more revolutionary change to a LAN than DHCP. A DHCP server and forwarding agents can allow you to set things up so that you can unplug a client computer from one network or subnet and plug it into another and have it come alive immediately, it having been reconfigured automatically. In conjunction to Dynamic DNS, it could automatically be given its same name in its new place. VLAN-capable LAN equipment with dynamic VLAN assignment allows you to configure things so a client computer can be plugged into any port and have the same IP number (as well as name) and be on the same subnet. The VLAN-capable network either has its own configuration that lists which MAC addresses are to belong to each VLAN, or it makes the determination from the source IP address of the IP packets that the client computer sends. Some differences in the two approaches:
§ DHCP handles changes by reconfiguring the client while a VLAN-capable network handles it by reconfiguring the network port the client is moved to.
§ DHCP dynamic reconfiguration requires a DHCP server, forwarding agent in each router, and DHCP capability in each client's TCP/IP support. The analogous capability in VLANs requires that all hubs throughout the network be VLAN-capable, supporting the same VLAN scheme. To this point VLAN support is proprietary with no vendor interoperability, but standards are being developed.
§ DHCP can configure a new client computer for you while a VLAN-capable network can't.
§ DHCP is generally aimed at giving "easy moves" capability to networks that are divided into subnets on a geographical basis, or on separate networks. VLANs are generally aimed at allowing you to set up subnets on some basis other than geographical, e.g. instead of putting everyone in one office on the same subnet, putting each person on a subnet that has access to the servers that that person requires.
There is an issue with trying to use DHCP (or BOOTP) and VLANs at the same time, in particular, with the scheme by which the VLAN-capable network determines the client's VLAN based upon the client computer's source IP address. Doing so assumes the client computer is already configured, which precludes the use of network to get the configuration information from a DHCP or BOOTP server.
Q. What is DHCP relay Agent?
A. DHCP Relay Agent component is a Bootstrap Protocol (BOOTP) relay agent that relays Dynamic Host Configuration Protocol (DHCP) messages between DHCP clients and DHCP servers on different IP networks.
Q. How does DHCP relay agent work?
A. A DHCP relay agent is an agent program or component responsible for relaying DHCP & BOOTP (Bootstrap Protocol) broadcast messages between a DHCP server and a client across an IP router.
A DHCP relay agent supports DHCP/BOOTP message relay as defined in RFC (Request for Comment) 1541 & 2131. The DHCP relay agent service is managed using Routing & Remote Service.
Q. DHCP User Class and Vendor Class Options?
A. DHCP provides support for a host of new features. The user-specified and vendor-specified DHCP options—features that let administrators assign separate options to clients with similar configuration requirements. For example, if DHCP-aware clients in your human resources (HR) department require a different default gateway or DNS server than the rest of your clients, you can configure DHCP Class IDs to distribute these options to HR clients. The options that Class IDs provide override any scope or global default options that the DHCP server typically assigns.
Q. Option Classes?
A. The two option class types: User Class and Vendor Class. User Classes assign DHCP options to a group of clients that require similar configuration; Vendor Classes typically assign vendor-specific options to clients that share a common vendor type. For example, with Vendor Classes you can assign all Dell computers DHCP options that are common to those machines. The purpose of option classes is to group DHCP options for similar clients within a DHCP scope.
Q. What is Super scope?
A. A range of IP addresses that span several subnets. The DHCP server can assign these addresses to clients that are on several subnets.
A. A super-scope is actually a collection of individual scopes. When you group different scopes together into a single superscope, you can do the following:
- Place DHCP clients from multiple network IDs on the same physical segment
- Allow remote DCHP clients from multiple network IDs to obtain an address from a DHCP Server
- Place multiple DHCP Servers on the same physical segment, with each DCHP Server being responsible for a different scope.
The superscope will allow the DHCP Server to answer requests from DHCP clients from different network IDs.
Q. What is Multicast?
A. A range of class D addresses from 224.0.0.0 to 239.255.255.255 that can be assigned to computers when they ask for them. A multicast group is assigned to one IP address. Multicasting can be used to send messages to a group of computers at the same time with only one copy of the message. The Multicast Address Dynamic Client Allocation Protocol (MADCAP) is used to request a multicast address from a DHCP server.
A. A DHCP lease is the amount of time that the DHCP server grants to the DHCP client permission to use a particular IP address. A typical server allows its administrator to set the lease time.
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