Dynamic Host Configuration Protocol

Mahmmoud A. Mahdi

Creating a DHCP infrastructure

DHCP allows you to assign IP addresses, subnet masks, and other configuration information to client computers on a local network.

When a DHCP server is available, computers that are configured to obtain an IP addresses automatically request and receive their IP configuration from that DHCP server upon booting.

DHCP Basics

Dynamic Host Configuration Protocol Automatic configuration▪ IP address▪ Subnet mask▪ Other Information

Requires DHCP Server Windows Server 2008 Role

Advantages of DHCP

Configuration of large and even midsize networks is much simpler. Once you enter the IP configuration information in one place(the

server) it’s automatically propagated to clients, eliminating the risk that a user will misconfigure some parameters and require you to fix them.

IP addresses are conserved because DHCP assigns them only when requested.

IP configuration becomes almost completely automatic. Allows a preboot execution environment (PXE) client to get a

TCP/IP address from DHCP. PXE clients (also called Microsoft Remote Installation Services (RIS)

clients) can get an IP address without needing to have an operating system installed. This allows RIS clients to connect to a RIS server through the TCP/IP protocol and download an operating system remotely.

Disadvantages of DHCP

DHCP can become a single point of failure for your network.

If the DHCP server contains incorrect information, the misinformation will automatically be delivered to all your DHCP clients. To fix the problem, you might have to visit each machine and

reconfigure it. If you want to use DHCP on a multisegment network,

you must put either a DHCP server or a rely agent on each segment,

or you must ensure that your router can forward Bootstrap Protocol (BOOTP) broadcasts.a

Exam objectives:

Configure dynamic host configuration protocol (DHCP).

Lessons:

1. Installing DHCP server.2. Configuring a DHCP server.

Lesson 1Installing DHCP server.

Lesson 1: Installing a DHCP server

After this lesson you will be able to: Deploy a DHCP server. Configure a server DHCP scope. Configure DHCP scope options.

Understanding DHCP address assignment

The negotiation between a DHCP client and DHCP server occurs in four stages:

1. Broadcast DHCP Discover.2. Respond with DHCP Offer.3. Respond with DHCP Request.4. Confirm with DHCP Ask.

The DHCP address assignment process

1. Broadcast DHCPISCOVER2. Respond with DHCPOFFER

3. Respond with DHCPREQUEST4. Confirm with DHCPACK

Understanding address leases

Every DHCP server maintains a database of addresses that the server can distribute to clients.

When a DHCP server assign a computer an address, it assigns that address in the form of a lease that lasts six or eight days by default.

The DHCP server keeps tracks of leased addresses so that no address is assigned to two clients.

To prevent an IP address from being indefinitely assigned to a client that has disconnected from the network , DHCP server reclaim addresses at the end of the DHCP lease period. If the DHCP server is online▪ Accept the renewal

If the DHCP server is not available▪ DHCP Client tries to renew the DHCP lease again after half the

remaining lease period If the DHCP server is not available after 87.5% of lease time

has elapsed▪ DHCP client attempts to locate a new DHCP server

If the DHCP client shuts down normally, or if an administrator runs the command ipconfig/release The client sends a DHCP release message to the DHCP server. The DHCP server marks the IP address as available and can

reassign it to a different DHCP client.

If the DHCP client disconnects suddenly from the network (does not have the opportunity to send a DHCP release message). The DHCP server will not assign the IP address to a different

client until the DHCP lease expires.

Ipconfig Lease Options

ipconfig /renew Instructs the DHCP client to request a lease renewal. ▪ If the client already has a lease, it requests a renewal from the

server that issued the current lease.▪ if the client doesn’t currently have a lease, It initiates the DHCP

mating dance, listens for lease offers, and chooses one it likes. ipconfig /release

Forces the client to immediately give up its lease by sending the server a DHCP release notification.

ipconfig /setclassid classID Sets a new class ID for the client.

Understanding DHCP Scopes

Scope: A range of IP addresses must be defined at the

DHCP server. Defines a single physical subnet on the network to

which DHCP services are offered. Example:▪ If you have two subnets ▪ 10.0.1.0/24 and 192.168.10.0/24

▪ DHCP server:▪ directly connect to each subnet▪ define a scope for each subnets & associated address ranges

Superscope Enables the DHCP server to provide addresses from

more than one scope to clients on the same physical subnet.

Helpful when clients within the same subnet have more than one IP network and thus need IPs from more than one address pool.

Microsoft’s DHCP snap-in allows you to manage IP address assignment in the superscope, though you must still configure other scope options individually for each child scope.

Exclusions and Reservations Exclusions are IP addresses within the range that you never want

automatically assigned. Reservations are IP addresses within the range for which you

want a permanent DHCP lease. They essentially reserve a particular IP address for a particular device.

Address Pool The range of IP addresses that the DHCP server can assign is

called its address pool DHCP Relay Agent

If no DHCP server is available on the client’s network, you can use a DHCP relay agent to forward DHCP broadcasts from the client’s network to the DHCP server.

Exam Tip

You should understand what a superscope is for the 70-642 exam.

Understanding DHCP Options

DHCP options provide clients with additional configuration parameters Such as DNS or WINS server address.

More than 60 standard DHCP options are available

The most common DHCP Options, For an IPv4 configuration are: 003 Router:

▪ Used to provide a list of available routers or default gateways on the same subnet 006 DNS Servers:

▪ Used to provide a list of DNS servers 015 DNS Domain Name:

▪ Used to provide the DNS suffix▪ Allows clients to perform dynamic DNS updates.

044 WINS/ NBNS Servers:▪ Used to configure the IP addresses of WINS servers.

046 WINS/ NBT Node type:▪ Used to configure the preferred NetBIOS name resolution method. There are four settings for node type:

▪ B node (0x1): Broadcast for NetBIOS resolution▪ P node (0x2): Peer-to-peer (WINS) server for NetBIOS resolution▪ M node (0x4): Mixed node (does a B node and then a P node)▪ H node (0x8): Hybrid node (does a P node and then a B node)

051 Lease:▪ Used to configure a special lease duration▪ Assigns a special lease duration only to remote access clients.▪ This option relies on user class information advertised by this client type.

Exam Tip

You need to understand these six DHCP options for the 70-642 exam.

Adding the DHCP server role

To install and configure a DHCP server on a computer running windows server 2008 Deploy a server on the physical subnet for which

you want to provide addressing. Be sure to assign the server a static IP address

that will be compatible with the address range planned for the local subnet.

Use the add roles wizard to add the DHCP server role on the computer.

Selecting the DHCP server role

Adding DHCP Server Role Wizard

The wizard presents you with the following configuration pages:

1. Select network connection bindings.2. Specify IPv4 DNS Server settings.3. Specify IPv4 WINS Server settings.4. Add or edit DHCP scopes.5. Configure DHCPv6 Stateless mode.6. Specify IPv6 DNS Server settings.7. Authorize DHCP Server.

(1) Selecting Network Connection Bindings

You specify the network adapter or adapters that the DHCP server will use to service clients.

If your DHCP server is multi-homed, this page gives you an opportunity to limit DHCP service to network only.

The select network connection bindings page

(2) Specifying IPv4 DNS Server Settings

Provides you an opportunity to configure the 015 domain names and the 006 DNS Servers options for all scopes that you will create on the DHCP server. The 015 DNS Domain Names option ▪ enables you to set a DNS suffix for the client connections obtaining an

address lease from the DHCP server. The 006 DNS Servers option ▪ enable you to configure a DNS server address list for the client

connections obtaining an addresses lease from the DHCP server.

The specify IPv4 DNS Server settings page

(3) Specifying IPv4 WINS Server Settings

Enables you to configure the 044 WINS/ NBNS Server option So you can assign a WINS server list to clients.

The specify IPv4 WINS Server settings page

(4) Adding DHCP Scopes

Enables you to define or edit scopes on the DHCP server. A scope is an administrative grouping of IP

addresses for computers on a subnet that use the DHCP service.

Each subnet can have only a single DHCP scope with a single continuous range of IP addresses.

The add or edit DHCP Scopes page

The add scope dialog box

The Process of Creating a Scope

Scope Name Use to label the scope as it appears in the DHCP console.

Starting and Ending IP Address You should use the consecutive addresses that makeup the subnet for

which you are enabling the DHCP service. You should be sure to exclude from this defined range any statically

assigned addresses for existing or planned servers on your network.▪ For example, on the same subnet you need to assign a static IP address to the local DHCP

server, router (default gateway), and any DNS servers, WINS servers, and domain controllers.

To exclude addresses, you can simply choose to limit the scope range so that it does not include any of static addresses assigned to servers.▪ For example, in the subnet 192.168.0.0/24 you can keep the addresses 192.168.0.1 through

192.168.0.20 for your statically addressed servers. ▪ You can then define the addresses 192.168.0.21 through 192.168.0.254 as the range for the

subnet’s DHCP scope.

Subnet Mask The subnet mask that will be assigned to DHCP clients that receive an address

lease through this scope. Be sure to choose the same subnet mask as the one configured for the DHCP

server itself. Default Gateway (optional)

Enables you to configure the 003 router option. Which assigns a default gateway address to the DHCP clients that receive an address lease through this scope.

Subnet Type Allows you to assign one of two lease durations to the scope. By default, the scope is set to the wired subnet type, which configure a lease

duration of six days. The alternative setting is wireless, for which the lease duration is eight hours.

Activate This Scope A scope will lease out addresses only if it is activated.

(5) Configuring DHCPv6 Stateless Mode

DHCPv6 refers to DHCP for IPv6 Stateless mode

the default addressing mode for IPv6 hosts in which addresses are configured without the help of a DHCP server while options can still be obtained from the DHCP server.

On the configure DHCPv6 Stateless Mode page you can disable stateless mode on the DHCP server and

enable to response to IPv6 hosts that has been enabled for stateful addressing.

Disable stateless addressing on the DHCP server you will later need to create a scope for an IPv6 address

range by using the DHCP console.

Configuring DHCPv6 stateless mode

Creating a scope for DHCPv6 clients

Note: Enabling stateful addressing for IPv6 hosts

To enable stateful addressing on an IPv6 host type the following command:

▪ netsh interface ipv6 set interface interface_name managedaddress=disabled

To enable the IPv6 host to obtain DHCP options from a DHCPv6 server type the following command:▪ netch interface ipv6 set interface interface_name

otherstateful=enabled

Exam Tip:

It is unlikely that you will see any questions about DHCPv6 on the 70-642 exam.

(6) Configuring IPv6 DNS Server Settings

When you leave the enable DHCPv6 Stateless Mode for this Server option selected The configure IPv6 DNS Server Setting page

appears.

(7) Authorizing DHCP Server

Gives you an opportunity to authorize a DHCP server for use in an Active Directory domain.

In Active Directory domain environments A DHCP server will not issue IP addresses to

clients unless the server is authorized. Requiring servers to be authorized reduces the

risk that a user will accidentally or intentionally create a DHCP server that assigns invalid IP address configurations to DHCP clients.

Authorizing a DHCP server

A DHCP Server that needs to be authorized

Quick Check

(fill in the blanks.) Before a DHCP server in a domain environment can lease addresses from an existing scope to any DHCP clients, you first need to……………the server and …………the scope.

Quick Check Answer: Authorize Activate

Correcting Client Problems

Problems with DHCP configurations can show up on the client side.

These problems might include the following: The client fails to obtain an IP address. There are address conflicts. The client obtains an address from the wrong

scope.

Repair Connection

When clicked, the Repair button performs a series of specific actions: If the network connection is configured as a DHCP client, the current

settings are released, and a DHCPREQUEST message is broadcast in an attempt to obtain new IP settings. ▪ This is the functional equivalent to performing an ipconfig /release and

ipconfig /renew. The ARP cache is flushed, as with the arp -d * command. The NetBIOS cache is flushed, as if the nbtstat -R command. The DNS cache is flushed, as if ipconfig /flushdns command. The client’s NetBIOS name and IP address are reregistered with the

WINS server, as if nbtstat -RR was entered. The client is reregistered with DNS as if ipconfig /registerdns

command.

Exam Essentials

Understand the four stages of the DHCP process Know how to install and authorize a DHCP server Know how to create a DHCP scope Understand how relay agents help with multiple

physical network segments Understand the difference between exclusions and

reservations Understand what a IPv4 superscope is used for Understand how to integrate IPv4 Dynamic DNS with

DHCP Understand how to troubleshoot DHCP problems

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