IP Addressing Facts in Designing Network Infrastructure

The following table lists the default IP addressing classes and masks:

Class	Address Range	Default Mask
A	1.0.0.0 to 126.255.255.255	255.0.0.0
B	128.0.0.0 to 191.255.255.255	255.255.0.0
C	192.0.0.0 to 223.255.255.255	255.255.255.0
D	224.0.0.0 to 239.255.255.255	(multicast addresses)
E	240.0.0.0 to 255.255.255.255	(experimental addresses)

You should also know the following address ranges that are reserved for private addresses. Use these addresses on a private network that is connected to the Internet through a network address translation (NAT) router.

• 10.0.0.0 to 10.255.255.255
• 172.16.0.0 to 172.31.255.255
• 192.168.0.0 to 192.168.255.255

Keep in mind the following facts about IP addresses:      

• The first address in a range on the subnet is the subnet address. Typically, this address is not assigned to hosts.
• The last address in a range on the subnet is the broadcast address. Typically, this address is not assigned to hosts.

Use the table as a shortcut guide to subnetting. Tip: Look for patterns in the table so you can easily reproduce the table at any time.

Masked Bits	Mask Value	Number of Subnets*	Number of Hosts/Subnet**	Approximate*** Number of Hosts/Subnet
/20	255.255.240.0	N/A	4096	4000
/21	255.255.248.0	N/A	2048	2000
/22	255.255.252.0	N/A	1024	1000
/23	255.255.254.0	N/A	512	500
/24	255.255.255.0	1	256	250
/25	255.255.255.128	2	128	125
/26	255.255.255.192	4	64	60
/27	255.255.255.224	8	32	30
/28	255.255.255.240	16	16	15

*The number of subnets value is important when you need to subdivide a Class C address range into multiple subnets.
**Remember to subtract two from these numbers to arrive at the total number of addresses that can be assigned to hosts (subtract one for the subnet address and one for the broadcast address.
***Use this number as a quick estimate of the number of possible hosts.

DHCP Design Guidelines

In a network with multiple subnets, a major concern is answering DHCP broadcast requests from all subnets. There are three ways to configure DHCP to allow clients on all subnets to get an IP address from a DHCP server.

Method	Considerations
Place a DHCP server on each subnet	The local DHCP server answers DHCP requests for its local subnet only. This solution could increase costs as you must configure a DHCP server for each subnet.
Enable BootP forwarding	Use this method if you choose to have only one DHCP server on the routed network. Place the DHCP server on the subnet with the most hosts.
Use relay agents	Configure a relay agent on each subnet that does not have a DHCP server to forward DHCP broadcasts to the remote DHCP server.

To provide fault tolerance and improve DHCP performance, you can:

• Configure split scopes. In a split scope, two DHCP servers service each subnet.
• Configure the full range of addresses in the scope on both DHCP servers.
• Exclude different ranges of addresses on each server. With both servers on the same subnet, exclude 50% of the addresses on each server. With one DHCP server on the local network, exclude addresses using the 80/20 (or 75/25) rule (exclude 80% on the remote server).
• Add relay agents to allow DHCP broadcasts to travel to remote subnets in case the local DHCP server goes down.
• Implement a DHCP cluster. Windows Server 2003 DHCP is cluster-aware. When one server goes down, the other server(s) can take over.

To decrease the time that the DHCP server takes to respond to client requests, consider the following to improve performance:

• Create a multihomed DHCP server. By doing so, the DHCP server services multiple subnets without forwarding DHCP requests through the router. A multihomed server will decrease router traffic and decrease the time it takes for clients on the remote subnet to receive their IP addresses.
• Upgrade DHCP server components. Upgrading the CPU, RAM, and network cards all affect DCHP performance, but because DHCP is a disk-intensive service, the most important upgrade is to improve disk access.
• Add additional DHCP servers. By doing so, you spread the workload between two computers.
• Modify lease lengths. Increasing the lease time means clients will not request an address as often. This reduces network traffic due to lease broadcasts, and reduces the workload on the DHCP server.

Members of the Enterprise Admins group can authorize DHCP servers (members of the Domain Admins group can administer, but not authorize, DHCP servers).