DHCP IP Address Allocation

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Questions and Answers

What happens when a client requests an IP address from a DHCP server?

The client assigns its own IP address

The DHCP server generates a new IP address

The DHCP server retrieves it from its storage and delivers it to the client (correct)

The DHCP server assigns a permanent IP address to the client

Which type of allocation is best suited for giving IP addresses to servers?

Static allocation

Manual allocation (correct)

Automatic allocation

Dynamic allocation

What is the main advantage of Dynamic allocation?

It allows for efficient management of a pool of network addresses (correct)

It assigns permanent IP addresses to clients

It is the simplest method of allocating IP addresses

It ensures that servers have permanent IP addresses

What happens when a client's lease expires?

The client's IP address is released back to the DHCP server Signup and view all the answers

What is the purpose of a lease in Dynamic allocation?

To allow the client to use the IP address for a limited period of time Signup and view all the answers

What is the purpose of the DHCPREQUEST message sent by the client?

To renew the lease of the current address Signup and view all the answers

What happens if the client does not receive an answer from the DHCP server until time2 has passed?

It enters the REBINDING state and sends a multicast message to all servers Signup and view all the answers

What can a client do when its lease is about to expire?

Extend its lease with subsequent requests Signup and view all the answers

What is the default value of time1?

Half of the lease time of the current address Signup and view all the answers

What is an advantage of using Dynamic allocation in a network with limited addresses?

It allows for efficient management of a pool of network addresses Signup and view all the answers

What is the main difference between Dynamic and Manual allocation?

Dynamic allocation assigns temporary IP addresses, while Manual allocation assigns permanent IP addresses Signup and view all the answers

What happens if the client gets a new address but not its previous address?

It continues to work, but not with the current web parameters, and must inform the users about this Signup and view all the answers

What is the purpose of the DHCP server?

To supply DHCP clients with the configuration parameters defined in the Host Requirements RFCs Signup and view all the answers

What does a client do with its IP address when it no longer needs it?

It releases the IP address back to the DHCP server Signup and view all the answers

What is the shortest waiting time before sending DHCPREQUEST again?

One minute Signup and view all the answers

What is the primary task of a DHCP server?

To provide a permanent storage of network parameters for network clients Signup and view all the answers

What is the format of the DHCP storage model?

A per-host list of entries of the form: key = value Signup and view all the answers

What happens if the client gets its previous address?

It continues to work normally Signup and view all the answers

How does a client request its configuration parameters from a DHCP server?

By sending a request message to the server Signup and view all the answers

What is the default value of time2?

0.875x(lease time) Signup and view all the answers

What is the purpose of the options field in a DHCP response message?

To carry the configuration parameters for the client Signup and view all the answers

What happens if the client does not get an address at all?

It stops its work and goes back to INIT state Signup and view all the answers

What is the advantage of using default values defined in the Host Requirements RFCs?

To reduce the number of parameters delivered from the server to the client Signup and view all the answers

What is the purpose of the parameter request list option?

To allow clients to request only those parameters required by the client Signup and view all the answers

What is the basis of the DHCP message format?

The format of BOOTP messages Signup and view all the answers

Why is the DHCP message format based on BOOTP messages?

To ensure compatibility with BOOTP relay agents and BOOTP clients Signup and view all the answers

What is the purpose of the DHCP protocol?

To provide a dynamic configuration of network parameters for network clients Signup and view all the answers

What is the purpose of the 'xid' field in a DHCP message?

To associate the request message with its response Signup and view all the answers

What is the length of the 'hlen' field in a DHCP message?

1 byte Signup and view all the answers

What is the purpose of the 'ciaddr' field in a DHCP message?

To specify the client's IP address Signup and view all the answers

What is the length of the 'chaddr' field in a DHCP message?

16 bytes Signup and view all the answers

What is the purpose of the 'hops' field in a DHCP message?

Optionally used by relay agents when booting via a relay agent Signup and view all the answers

What is the purpose of the 'flags' field in a DHCP message?

Flags Signup and view all the answers

What is the purpose of the 'yiaddr' field in a DHCP message?

To specify the 'your' IP address Signup and view all the answers

What is the length of the 'sname' field in a DHCP message?

64 bytes Signup and view all the answers

What is the purpose of the 'siaddr' field in a DHCP message?

To specify the server's IP address Signup and view all the answers

What is the first message sent by the DHCP client in the client-server negotiation process?

DHCPDISCOVER Signup and view all the answers

What is the purpose of the DHCPREQUEST message?

To select one server from which to request configuration parameters Signup and view all the answers

What happens when the client receives a DHCPNAK message?

The client restarts the configuration process Signup and view all the answers

What is the role of the DHCPOFFER message?

To offer configuration parameters to the client Signup and view all the answers

How many servers can be selected by the client in the DHCP negotiation process?

Only one server Signup and view all the answers

What is the purpose of the DHCPRELEASE message?

To relinquish the lease on a network address Signup and view all the answers

What is the final message sent by the server in the DHCP negotiation process?

DHCPACK Signup and view all the answers

What happens when the server receives a DHCPRELEASE message?

The server marks the lease as free Signup and view all the answers

What is the purpose of the DHCPACK message?

To commit the configuration parameters Signup and view all the answers

What happens when a DHCP client sends a request to a non-authoritative server?

The server ignores the request, leading to an implementation-dependent time out for the client Signup and view all the answers

What is the purpose of the DHCPOFFER message?

To reserve an IP address for the client and send an offer Signup and view all the answers

What happens when the client receives a DHCPOFFER message?

The client broadcasts a DHCPREQUEST message to all servers Signup and view all the answers

What information is included in the DHCPOFFER message?

The client's MAC address, IP address, subnet mask, lease duration, and server's IP address Signup and view all the answers

What is the purpose of the DHCPREQUEST message?

To accept an offer made by the DHCP server Signup and view all the answers

What is the final phase of the configuration process?

The DHCP acknowledgement phase Signup and view all the answers

What happens when a client's IP lease is still valid on a different subnet?

The client may request its last-known IP address. Signup and view all the answers

Which type of server may grant a client's request for its last-known IP address?

Authoritative server Signup and view all the answers

What is the purpose of a client's broadcast in the DHCP discovery phase?

To find available DHCP servers. Signup and view all the answers

What may a client start after obtaining an IP address to prevent IP conflicts?

An address resolution query. Signup and view all the answers

Which UDP port is used by the DHCP client side?

68/udp Signup and view all the answers

What is the destination of a client's UDP packet in the DHCP discovery phase?

255.255.255.255 Signup and view all the answers

What is the primary purpose of a DHCPINFORM request message?

To obtain local configuration parameters without allocating a new address Signup and view all the answers

What happens when a client reuses a previously allocated network address?

The client includes its network address in the 'requested IP address' option Signup and view all the answers

What is the purpose of a DHCPOFFER message?

To offer configuration parameters to the client Signup and view all the answers

What message is sent by the client to locate available DHCP servers?

DHCPDISCOVER Signup and view all the answers

What is the purpose of the DHCPREQUEST message?

To decline offers from all other servers Signup and view all the answers

What is the response sent by the server when a client reuses a previously allocated network address?

DHCPACK Signup and view all the answers

What is the purpose of the DHCPDISCOVER message?

To locate available DHCP servers Signup and view all the answers

What is the response sent by the server when a client sends a DHCPINFORM request message?

DHCPACK Signup and view all the answers

Study Notes

DHCP Allocation Methods

There are three methods of DHCP allocation: Manual, Automatic, and Dynamic allocation.
Manual allocation is best suited for giving IP addresses to servers, as it guarantees a permanent and known location in the network.
Dynamic allocation is the most interesting method, as it involves assigning, reclaiming, and reusing network addresses.
It's useful in cases where there is a limited amount of network addresses, and computers temporarily connect and disconnect to the network.

Dynamic Allocation

The client requests a network address for a limited period of time (lease).
The DHCP server allocates an address, marks it as 'used', and notifies the client about the address and lease time.
The client can extend its lease, ask for a permanent assignment, or release the address back to the server before the lease expires.

Renewing and Acquiring Addresses

The client holds two times in its memory: time1 and time2.
Time1 is when the client starts to ask its server for renewing the lease, and time2 is when the client starts to ask other servers for an address.
If the client doesn't get an answer, it waits half of the time left before sending DHCPREQUEST again.
The shortest waiting time is one minute.

Configuration Parameters Delivery

The DHCP server supplies clients with configuration parameters defined in the Host Requirements RFCs.
The server provides a permanent storage of network parameters for clients.
The storage model is a set of key-value pairs for each client, where the key is a unique identifier and the value contains the configuration parameters.

Message Format

The DHCP message format is based on the BOOTP message format.
The message format includes fields such as op, htype, hlen, hops, xid, secs, flags, ciaddr, yiaddr, siaddr, giaddr, chaddr, sname, and file.

Client/Server Model

The client and server negotiate in a series of messages to get the parameters the client needs.
The client broadcasts a DHCPDISCOVER message, and each server responds with a DHCPOFFER message.
The client chooses one server and broadcasts a DHCPREQUEST message, and the selected server responds with a DHCPACK message.

Steps of Communication

The client broadcasts a DHCPDISCOVER message.
Each server responds with a DHCPOFFER message.
The client chooses one server and broadcasts a DHCPREQUEST message.
The selected server responds with a DHCPACK message.
The client receives the DHCPACK message and is configured.
If the client receives a DHCPNAK message, it restarts the configuration process.
The client may choose to relinquish its lease on a network address by sending a DHCPRELEASE message.
The server receives the DHCPRELEASE message and marks the lease as free.

DHCP Fundamentals

DHCP (Dynamic Host Configuration Protocol) is a network management protocol used to dynamically assign IP addresses to devices on a network.
DHCP automates and centrally manages IP configurations, replacing manual assignments by network administrators.

DHCP Operations

DHCP operations fall into four basic phases: IP lease request, IP lease offer, IP lease selection, and IP lease acknowledgement.

IP Lease Request

A client broadcasts a request for an IP address to available DHCP servers on the local physical subnet.
The client can request its last-known IP address, and the server may grant the request if it is still valid.

IP Lease Offer

The DHCP server responds with a DHCPOFFER message containing the client's MAC address, IP address, subnet mask, lease duration, and server's IP address.
The server determines the configuration based on the client's hardware address (CHADDR field).

IP Lease Selection and Acknowledgement

The client accepts an offer by broadcasting a DHCPREQUEST message, and other DHCP servers withdraw their offers.
The DHCP server responds with a DHCPACK packet, including the lease duration and any other requested configuration information.
The TCP/IP configuration process is complete after the client receives the DHCPACK packet.

DHCP Message Types

DHCPDISCOVER: client broadcast to locate available servers
DHCPOFFER: server response to DHCPDISCOVER with configuration offer
DHCPREQUEST: client message to servers for requesting offered parameters or confirming correctness of previously allocated address
DHCPACK: server response to DHCPREQUEST, including lease duration and configuration information

Additional DHCP Features

A client can reuse a previously allocated network address by including it in the 'requested IP address' option in the DHCPREQUEST message.
A client can obtain local configuration parameters without allocating a new address using a DHCPINFORM request message.

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Description

Learn about manual and dynamic allocation of IP addresses in DHCP, including its suitability for servers and temporary assignments.