Key message: Students will be able to identify when a service allocates an IP address automatically, and when the system self-configures an IP address, as in Automatic Private IP Addressing (APIPA) and link-local addresses.
Discussion prompt: Ask students if they recognize the IP address 169.254.1.2.
Answer: It is an APIPA address
On IPv4 networks, addresses can be assigned to hosts in three ways:
Manually, using static address assignment
Automatically, using DHCP, if a DHCP server is present on the subnet (or a DHCP relay agent configured on the subnet)
Automatically ,using Automatic Private IP Addressing (APIPA), which randomly assigns the host an address from the range 169.254.0.0 to 169.254.255.255 with subnet mask 255.255.0.0
DHCPv4 provides an automated way to distribute and update IP addresses and other configuration information on a network. A DHCP server provides this information to a DHCP client through the exchange of a series of messages, known as the DHCP conversation or the DHCP transaction. If the DHCP server and DHCP clients are located on different subnets, a DHCP relay agent is used to facilitate the conversation.
The DHCP architecture consists of DHCP clients, DHCP servers, and DHCP relay agents on a network. The clients interact with servers using DHCP messages in a DHCP conversation to obtain and renew IP address leases.
DHCP Client Functionality
A DHCP client is any network-enabled device that supports the ability to communicate with a DHCP server in compliance with RFC 2131, for the purpose of obtaining dynamic leased IP configuration and related optional information.
Automatic IP Configuration
DHCP supports Automatic Private IP Addressing (APIPA), which enables computers that are running Windows 2000, Windows XP, and Windows Server 2003 to configure an IP address and subnet mask if a DHCP server is unavailable at system startup and the Automatic private IP address Alternate Configuration setting is selected. This feature is useful for clients on small private networks, such as a small-business office or a home office.
The DHCP Client service on a computer that is running Windows XP and Windows Server 2003 uses the following process to auto-configure the client:
The DHCP client attempts to locate a DHCP server and obtain an IP address and configuration.