DHCP in Windows Server 2003 D ynamic - Means that clients IP address may change H ost - Indicates that this is a system for clients, e.g. XP machines C onfiguration - A clue that you are in charge of the options, e.g. DNS Server P rotocol - The rules controlling the flow of packets between client and server Tutorial for DHCP in Windows 2003 Benefits of DHCP All clients and servers need an IP address on a TCP/IP network. How will you configure those dotty dot numbers on your TCP/IP property tabs? Manually, or automatically via DHCP? Let us investigate what advantages an automatic DHCP service has over the manual alternative. - DHCP needs much less effort - manual configuration is boring and labour intensive. Above all, DHCP options give a sense of central control.
- Easy to update a default gateway or DNS server's IP address. Manual changes would be a nightmare, you would have to visit every machine.
- No IP duplicate addresses. Provided you configure the DHCP scopes intelligently, there will never be another 'Duplicate IP address' problem to eat into your valuable time.
Strategies for the clients and servers 10 years ago, when I first saw DHCP, I thought that you would need one DHCP server on each subnet - wrong. What I now recommend as a default, is two DHCP servers for the whole company. For those subnets without a DHCP you configure a DHCP Relay Agent. If you have two DHCP servers, then provide redundancy by splitting each scope so that each DHCP server gets a non-overlapping range. For example: Server A: 10.10.56.1 to 10.10.56.120
Server B: 10.10.56.121 to 10.10.56.254. Each scope has a class C Subnet Mask /24 (255.255.255.0) Strategies for the servers What are you going to do about the IP addresses for the servers themselves? - Configure static IP addresses, then EXCLUDE a range from the scope.
- Select static IP addresses, but use a different range.
- Choose DHCP configuration, even for file and print servers. (Except the DHCP server itself.)
If you try the strategy of DHCP address for file and print servers, consider a RESERVATION for each server. Slowly I am warming to this DHCP idea, the killer advantage is that you can set DNS and Router options even for the servers. Let me elaborate, if you set server IP addresses manually, but then you change the default gateway, you may forget to change the servers default gateway. The result would be a loss of what ever service the servers were providing. However, if the servers have a reserved IP address then they come under the umbrella of your scope options and so there would be no extra work, and no loss of service. Summary DHCP is now a well established strategy for providing computers with IP addresses. However, it is full of surprises and hidden treasures, take the time to develop your DHCP tactics, then explore the properties of both the DHCP server icon and the scopes. I have a series of tutorials to help you. Install and Configure DHCP in Windows Server 2003 As services go, DHCP is easy to both install and configure. However, because there are so many settings in so many places, I am willing to bet that my tutorial will unearth at least one new option that will improve your DHCP performance. Tutorial to Install and Configure DHCP Servers DHCP Install  This tutorial will guide you through the steps needed to get your DHCP server installed and configured correctly. Let us begin with a straightforward job to install DHCP. Get your Windows Server CD ready, then navigate to: Add Remove Programs, Windows Components, Networking Services. Whilst adding the DHCP service is easy, configuring the scope options needs thought. For instance, if you make a mistake with the subnet mask, you cannot amend that scope, you would have to delete and start afresh. However, you can add and change the options such as Type 006 DNS server, or Type 015 Domain name. DHCP Address Leases Lease is a good name for a DHCP IP property. Take for example the 8 day default lease; if the client is shutdown for 2 days, when it restarts it will continue to have the same IP address. Halfway through their lease clients attempt to renew their lease. IPCONFIG /all will show you the lease, while /renew will do what it says, top up the lease. Only reduce the duration if you are short of IP addresses. For example, if you only have 250 IP addresses but 300 possible clients. It also makes sense to set short leases if you are likely to discontinue a scope in the near future. Here is a table summarising how a DHCP service results in clients getting an IP address. If you are interested in seeing these packets, use Network monitor to capture DHCP in action. Here are the classic 4 packets that clients exchange during a lease negotiation. Client | Server | | DHCPDiscover --> | <--- DHCPOffer | DHCPRequest --> | <--- DHCPack | | | | | DHCPInform Server check that it is Authorized in Active Directory | Note 1: DHCPRequest may seem strange, but it comes into play if there are two DHCP servers and both make an offer to a potential client. Note 2: DHCPack. Once in a blue moon you see DHCPNack this is a negative acknowledgement which mean, ' I do no know you'. The most likely cause of Nack is the client is trying to renew an IP address from the wrong DHCP server. Scope Options. Take the time to investigate Scope Options, this the most likely place that I will win my bet that you will find a new setting which will improve your network performance. These options can be set at the Scope Level, Server Level, Reservation Level or at the Class Level (Tricky). So find all four places and make up your mind which would be the best level for your network. Examples of DHCP Scope Options: - Router (Default Gateway), DNS Servers (006)
- Domain Name (015) WINS (044 and 046)
Classes (Advanced Tab) - Vendor Class - Windows 98 Machines
- User Class - Routing and Remote Access
- Creating your own User Class - See more here.
Address Reservation Reserving IP addresses is useful in two situations, for file and print servers and for important machines where leases are in short supply. How does DHCP know which machine to lease a particular IP? The answer is by its MAC address (also called NIC or Physical address). In Windows 2003 when you enter the MAC address DHCP strips out the hyphens if you absentmindedly include them amongst the HEX numbers. To find the MAC address ping the machine then type arp -a. Remember that you can set DHCP Options for the reservations; after all, that may have been the very reason why you decided to make reservations in the first place. Authorize - DHCP Server In a Windows Server 2003 (or 2000) domain all DHCP servers need to be authorized in Active Directory. This is an example of Microsoft's new security initiative, and an attempt to eliminate rogue DHCP servers set up junior administrators in a large company. So, you need to logon (or RunAs) a member of the Enterprise Admins group. Then right click the DHCP server icon, and Authorize. Incidentally, The RIS service also needs to be Authorized before it becomes active. Activate - DHCP Scope Even after you Authorize a server, each scope must be activated individually. So, right click the scope to activate (or deactivate). Keep your eye on the red or green arrows to judge your success. Note you may have to Refresh from the server icon, often pressing F5 is not enough. Thanks to http://computerperformance.co.uk/w2k3/services
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