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Distributed DHCP infrastructure

When you install at least one DHCP server on each of your subnets so that all of your clients have access to a local DHCP server.

Especially good to add to existing servers on each subnet since the traffic will be lighter.


Centralized DHCP infrastructure

All the DHCP servers are placed in a single location, and each subnet has a DHCP relay agent. Many routers have a built-in DHCP relay option, or you can use the Network Policy and Access Services Role.


DHCP relay agent

A software component that receives the DHCP broadcast traffic on a subnet and then sends it on to particular DHCP servers on one or more other networks. Disabled by default.

Basically the same as the original BOOTP really agent.


Hybrid DHCP infrastructure

One that uses multiple DHCP servers on different subnets, but does not necessarily require a DHCP server on every subnet. Some subnets have relay agents instead of DHCP servers.

(Ex: each LAN has a few DHCP servers and the rest of the subnets have relay agents, freeing up the WAN links)


The default lease interval for a Windows Server 2008 DHCP server is:

6 days (so renewal happens every 3 days)

Only increase the lease interval if you have plenty of unused addresses and if the computers don't move around much.


Three techniques for providing fault tolerance to DHCP servers:

- Splitting scopes
- Failover clustering
- Using standby servers


Splitting scopes

The most common method of providing DHCP fault tolerance. You create identical scopes on two DHCP servers, then give them opposite exclusion ranges. The most common ratio for scope splitting is 80/20 (the 80/20 rule). The 80% server is on the subnet it is servicing, while the 20% server is accessed through a relay agent (the 80% server will be reached first, unless it is too busy). A delay could be configured into the relay agent if desired.

Also, one server could be the 80% for 2 scopes and the 20% for two other scopes, and have them both reached by relay agent with delays.


Failover clustering

The DHCP service is replicated on two or more computers that use the same storage medium (like an iSCSI or Fibre Channel storage). The DHCP server on one of the servers is active and the rest are dormant until the active one fails and one of the others need to take over.

Usually overkill for DHCP servers.


Standby server

A computer with the DHCP Server role installed and configured, but not activated. An admin manually activates it if one of the DHCP servers fail. This is inexpensive because the standby servers can be fulfilling other roles in the meantime, but it is not automatic like failover clustering.


When you have two identical DHCP scopes on different servers, you must configure the servers to use:

Server-side address conflict detection by specifying a value for the Conflict Detection Attempts setting on the Advanced tab in the IPv4 Properties sheet.


In a DHCP configuration, a manually allocated address is called a:

Reservation. It is associated with the computer's MAC address.

Using DHCP reservations to assign permanent addresses ensures that another admin won't reserve the same address - all the addresses are managed by DHCP.


IPv6 unicast addresses assigned to registered computers are split into six variable-length sections:

- Format prefix: the type of address (provider-based unicast, multicast, anycast)

- Registry ID: identifies the Internet address registry assigned to the Provider ID.

- Provider ID: Identifies the ISP that assigned this portion of the address space to a particular subscriber.

- Subscriber ID: Identifies a particular subscriber to the service provided by the ISP specified in the Provider ID field

- Subnet ID: Identifies all or part of a specific physical link on the subscriber's network.

- Interface ID: Identifies a particular network interface on the subnet specified in the Subnet ID field.


Enable DHCPv6 stateless mode for this server:

IPv6 clients do not obtain addresses from the DHCP server, but they can obtain other TCP/IP configuration settings from the server.


Disable DHCPv6 stateless mode for this server:

IPv6 clients obtain addresses, as well as other TCP/IP configuration settings from the DHCP server.


DHCP relay agent hop-count threshold

Specifies the max number of relay agents the DHCP messages can pass through before being discarded. The default is four and the max is 16.


DHCP relay agent boot threshold

Specifies the time interval (in seconds) that the relay agent should wait before forwarding each DHCP message it receives. The default is 4. This enables you to control which DHCP server processes the clients for a particular subnet.


Host table

A list of names and their equivalent IP addresses, used in early TCP/IP addressing (before DNS).


Name resolution

Converting host names into IP addresses.


DNS consists of three elements:

- The DNS name space
- Name servers
- Resolvers


The DNS name space

A tree-structured namespace in which each branch of the tree identifies a domain. Each domain contains a collection of resource records that contain host names, IP addresses, and other information. Query operations are attempts to retrieve specific resource records from a particular domain.


DNS name servers

A DNS server is an application running on a server computer that maintains information about the domain tree structure and (usually) contains authoritative information about one or more specific domains in that structure. The application is capable of responding to queries for information about the domains for which it is the authority, and also of forwarding queries for information about other domains to other name servers. This enables any DNS server to access information about any domain in the tree.


DNS resolver

A client program that generates DNS queries and sends them to a DNS server for fulfillment. A resolver has direct access to at least one DNS server and can also process referrals to direct its queries to other servers when necessary.


Domain vs DNS domain:

Domain: a grouping of Windows computers and devices that are administered as a unit.

DNS domain: a group of hosts and possibly subdomains that represents a part of the DNS namespace.