chapter 1 DHCP Flashcards
Describe DHCP
DHCP is a communication protocol that lets network administrators centrally manage and automate the assignment of Internet Protocol (IP) addresses in an organization’s network.
Basically it frees a network administrator from having to manually configure each host connected to the network.
What is the Purpose of DHCP
To deliver host-specific configuration parameters from a DHCP server to a host.
Allocation of network addresses to hosts.
Give a detailed overview of DHCP (2)
DHCP is an application-layer protocol in the TCP/IP model.
DHCP is a UDP Service and it runs on port 67 and 68.
Name the 3 Mechanisms DHCP supports for IP address allocation (3)
Automatic allocation
Dynamic allocation
Manual allocation
Describe IP Address Allocation (3)
Manual Allocation: The administrator assigns a pre-allocated IP address to the client and DHCP only communicates the IP address to the device.
Automatic Allocation: DHCP assigns a static IP address permanently to a device, An IP address, once associated with a MAC address, is permanently associated with it until the server’s administrator intervenes.
Dynamic Allocation: DHCP dynamically assigns, or leases, an IP address from a pool of addresses for a limited period of time chosen by the server, or until the client tells the DHCP server that it no longer needs the address.
Name the Components of DHCP (3)
DHCP Server
DHCP Client
DHCP Relay Agent
Used to facilitate the conversation if the DHCP server and the DHCP client are on different IP subnet.
DHCP must: (4)
- Guarantee unique network address.
- Retain DHCP client configuration across a client reboot.
- Allow automated assignment of configuration parameters to new clients.
- Support fixed allocation of configuration parameters to specific clients.
Describe DHCP Messages DHCPDISCOVER DHCPOFFER DHCPREQUEST DHCPOFFER
DHCPDISCOVER - Client broadcast to locate available servers.
DHCPOFFER - Server to the client in response to a DHCPDISCOVER. The server uses the DHCPOFFER message to “offer” an IP address, lease time, and network configuration parameters to the client.
DHCPREQUEST Sent by the client to the server in response to a DHCPOFFER. The “server identifier” field of the DHCPREQUEST indicates which server the client has chosen to further interact with. All servers that sent the client a DHCPOFFER receive the DHCPREQUEST. The ones that are not chosen simply use the message as notification that they have not been chosen. The server that is chosen responds to the request, either with a DHCPACK or a DHCPNAK.
Describe DHCP Messages
DHCPACK
DHCPNAK
DHCPACK - Server to the client in response to a DHCPREQUEST. The DHCPACK indicates that the server “acknowledges” the request, and the DHCPACK message contains fields which indicate the IP address, lease time, and network configuration parameters that the client is being configured with.
DHCPNAK - Server to the client in response to a DHCPREQUEST. The DHCPNAK indicates that the server does not acknowledge the request, and does not agree to lease the specified IP address.
Describe DHCP Messages
DHCPRELEASE
DHCPDECLINE
DHCPRELEASE - Client to server to give up an IP address lease. If the client knows that it no longer needs an IP address, it should send the server a DHCPRELEASE.
DHCPDECLINE - Client to server in response to a DHCPACK. If the client receives a DHCPACK, but, for some reason, is not satisfied with the lease time and/or network parameters in the message, it can send the server a DHCPDECLINE indicating that it refuses to use the IP address.
4 steps of Dynamic allocation of a network address
Between a DCHP Client and DHCP Server.
- Discover
- Offer
- Request
- Acknowledge
Even flow diagram
2 Cases
Client-server interaction while allocating a new network address
Client-server interaction while reusing previously allocated network address
Describe DHCP Relay Agent (2)
Typically the clients and DHCP servers are not in the same network segment.
Network administrators can configure a local router to forward DHCP packets to a DHCP server from a different subnet.
Describe IP helper-address (Relay)
The router needs to be configured in such a way that whenever it receives a DHCP broadcast from a device that is not in the same network segment as the DHCP servers it forwards the packet to the DHCP servers.
On cisco routers you can configure the nearest interface to the device with ip helper-address command
Router(config)# interface fa0
Router(config-if)# ip helper-address 10.1.1.5
Describe Additional DHCP Parameters
In addition to IP address and subnet mask information, DHCP can provide the following protocol parameters: • Default Gateway • Domain Name and DNS servers • Time Servers • WINS servers
