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Windows Server 2012 : DHCP,IPv6 and IPAM - Exploring DHCP (part 1)

7/15/2014 4:23:50 AM

The Need for DHCP

Network administrators cannot expect end users and even IT personnel to be able to manually configure each network device’s IP address settings. Furthermore, many end users may not even have the permissions to change network configurations. Because of these and other challenges, DHCP services are required on most networks to enable network connectivity. Also, many devices do not provide an interface simple enough or readily available to configure network settings. DHCP provides a simple way to not only deliver IP addressing from a central administrative point, but it also allows the network administrators to control how these devices actually connect to the network and greatly enhance the management of these network-connected devices through this service.

Outlining DHCP Predecessors: RARP and BOOTP

Before the DHCP service was developed, two predecessors provided the first implementations of automated IP addressing. The first was the Reverse Address Resolution Protocol (RARP), and the second was the Bootstrap Protocol (BOOTP).

To understand RARP, an IT administrator should first understand the Address Resolution Protocol (ARP). Each network adapter, wired or wireless, has a unique address burned into it. This address never changes and it called the Media Access Control (MAC) address. The ARP stores IP address-to-MAC address information. For example, if you know the IP address of a system on the network, the ARP table will provide the corresponding MAC address associated with that IP address. On most systems and network devices, the ARP table is built dynamically based on previous and current connections, but only for systems on the same network segment. RARP tables, however, are the reverse in both the fact that they are not dynamically built and they are MAC-to-IP resolution.

The RARP service allows a newly connected system to broadcast its MAC address on the network and the RARP service will respond with the assigned IP address. This allows the new system to basically connect dynamically to the network. A few catches exist, however. The first catch is that the RARP administrator must first collect that new system’s MAC address and create an entry on the RARP table on the service with a corresponding IP address. The next catch is that RARP delivers a system an IP address but no other networking information, such as a subnet mask, router IP address, or DNS server or other networking options. The RARP service was limited to usage on a single flat network, but was useful in its time.

The next predecessor is the BOOTP service. The BOOTP service provided an IP address to clients requesting one, but did not require a predefined table of related MAC and IP addresses. BOOTP was designed to not only get a system connected, but to also provide additional information to systems looking to load or boot an operating system stored on the network. BOOTP is still used today for some network boot implementations but has been superseded by the DHCP service.

Examining the DHCP Client Service

The DHCP client service is the client-side service that requests an IP address from the network. Depending on the system’s network adapter configuration, the DHCP client service may be active or inactive and, if the client is leveraging network boot, can come in the form of a BOOTP or PXE client controlled by the system board. The Windows DHCP client service, however, is managed by the configuration stored within the Microsoft operating system and, furthermore, on each adapter. If the adapter senses a network connection and the IP address configuration is configured for automated IP addressing, the DHCP client service broadcasts the request for an IP address, and when the data is received from the server, the DHCP client service applies the lease information to the appropriate adapter and enables network communications. With the DHCP IP address lease, there is an important piece of information delivered, known as the lease duration. The lease duration informs the client how long the IP address can be used before the client must check back with the DHCP server to renew the lease or get a new lease. The DHCP client caches this information, and when the lease duration is nearly up or when the system is restarted or the network is reinitialized, the DHCP client contacts the DHCP server to ensure the lease can still be used so that it can be renewed or replaced with a new lease.

In addition, on Microsoft systems, the DHCP client service also manages the Dynamic DNS registration of the client if there is a Dynamic DNS server available. This is true unless the DHCP server service is mandating that DHCP leases have their dynamic DNS registration handled by the server itself.

Exploring the DHCP Server Service

The DHCP server service is the latest implementation of automated network addressing in use today. The DHCP server service can provide all the same functionality of a BOOTP service, but can also provide additional information to clients who are requesting an IP address. The DHCP server service provides a client an IP address in three steps:

1. DHCP client boots and broadcasts a DHCP IP request to all nodes on the local network.

2. A DHCP server on the local network receives the request and prepares to distribute an IP address to this client in the form of a DHCP IP address lease.

3. After the DHCP server has determined the right prerequisite information from that client request, it issues the client with a DHCP IP address lease, including additional DHCP lease options such as subnet mask, default gateway, and most likely, DNS server IP addresses.

Other -----------------
- Windows Server 2012 : DHCP,IPv6 and IPAM - Understanding the Components of an Enterprise Network
- Windows Server 2012 : Configuring IPv6/IPv4 interoperability (part 7) - ISATAP
- Windows Server 2012 : Configuring IPv6/IPv4 interoperability (part 6) - Configuring a DHCPv6 server, IPv6 transition technologies
- Windows Server 2012 : Configuring IPv6/IPv4 interoperability (part 5) - Stateless address autoconfiguration,Stateful address autoconfiguration
- Windows Server 2012 : Configuring IPv6/IPv4 interoperability (part 4) - IPv6 address assignment - Manual address assignment
- Windows Server 2012 : Configuring IPv6/IPv4 interoperability (part 3) - IPv6 address representation
- Windows Server 2012 : Configuring IPv6/IPv4 interoperability (part 2) - Default IPv6 functionality
- Windows Server 2012 : Configuring IPv6/IPv4 interoperability (part 1) - IPv6 concepts and terminology
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