Книга: Windows Server 2012 R2 Storage, Security, & Networking Pocket Consultant

Using dynamic IPv4 addressing and configuration

Using dynamic IPv4 addressing and configuration

A computer that uses dynamic IPv4 addressing and configuration is called a DHCPv4 client . When you start a DHCPv4 client, a 32-bit IPv4 address can be retrieved from a pool of IPv4 addresses defined for the network’s DHCP server.

The address is assigned to the client for a specified time period known as a lease . When the lease is approximately 50 percent expired, the client tries to renew it. If the client can’t renew the lease at that time, it tries again before the lease expires. If this attempt fails, the client tries to contact an alternate DHCP server. IPv4 addresses that aren’t renewed are returned to the address pool. If the client is able to contact the DHCP server but the current IP address can’t be reassigned, the DHCP server assigns a new IPv4 address to the client.

The availability of a DHCP server doesn’t affect startup or logon (in most cases). DHCPv4 clients can start and users can log on to the local computer even if a DHCP server isn’t available. During startup, the DHCPv4 client looks for a DHCP server. If a DHCP server is available, the client gets its configuration information from the server.

If a DHCP server isn’t available and the client’s previous lease is still valid, the client pings the default gateway listed in the lease. A successful ping tells the client that it’s probably on the same network it was on when it was issued the lease, and the client continues to use the lease as described previously. A failed ping tells the client that it might be on a different network. In this case, the client uses IPv4 autoconfiguration. The client also uses IPv4 autoconfiguration if a DHCP server isn’t available and the previous lease has expired.

IPv4 autoconfiguration works like this:

1. The client computer selects an IP address from the Microsoft-reserved class B subnet 169.254.0.0 and uses the subnet mask 255.255.0.0. Before using the IPv4 address, the client performs an Address Resolution Protocol (ARP) test to be sure that no other client is using this IPv4 address.

2. If the IPv4 address is in use, the client repeats step 1, testing up to 10 IPv4 addresses before reporting failure. When a client is disconnected from the network, the ARP test always succeeds. As a result, the client uses the first IPv4 address it selects.

3. If the IPv4 address is available, the client configures the NIC with this address. The client then attempts to contact a DHCP server, sending out a broadcast every five minutes to the network. When the client successfully contacts a server, the client obtains a lease and reconfigures the network interface.

As part of your planning, you need to consider how many DHCP servers should be installed on the network. Typically, you’ll want to install at least two DHCP servers on each physical network segment. Windows Server 2012 R2 includes DHCP failover for IPv4. DHCP failover enables high availability of DHCP services by synchronizing IPv4 address leases between two DHCP servers in one of two modes:

? Load Balance When you load balance the servers, you specify the percentage of the load each server should handle. Typically, you use a 50/50 approach to make each server equally share the load. You also could use other approaches, such as 60/40 to make one server carry 60 percent of the load and the other 40 percent of the load.

? Hot Standby With hot standby, one of the servers acts as the primary server and handles the DHCP services. The other acts as a standby server in case the primary fails or runs out of addresses to lease. A specific percentage of available IP addresses are reserved for the hot standby-5 percent by default.

The configuration of DHCP failover is simple and straightforward, and it does not require clustering or any advanced configuration. To configure DHCP failover, all you need to do is complete the following steps:

1. Install and configure two DHCP servers. The servers should be on the same physical network.

2. Create a DHCPv4 scope on one of the servers. Scopes are pools of IPv4 or IPv6 addresses you can assign to clients through leases.

3. When you establish the other server as a failover partner for the DHCPv4 scope, the scope is replicated to the partner.

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