IP Addressing Services Summary (25.3)
The following is a summary of each topic in the chapter and some questions for your reflection.
What Did I Learn in this Module? (25.3.1)
In data networks, devices are labeled with numeric IP addresses to send and receive data over networks. Domain names were created to convert the numeric address into a simple, recognizable name. The DNS protocol defines an automated service that matches resource names with the required numeric network address. The DNS protocol communications use a single format called a message. This message format is used for all types of client queries and server responses, error messages, and the transfer of resource record information between servers.
The DNS server stores different types of resource records that are used to resolve names. These records contain the name, address, and type of record. DNS uses the same message format between servers, consisting of a question, answer, authority, and additional information for all types of client queries and server responses, error messages, and transfer of resource record information.
DNS uses domain names to form the hierarchy. The naming structure is broken down into zones. Each DNS server maintains a specific database file and is only responsible for managing name-to-IP mappings for that small portion of the entire DNS structure. When a DNS server receives a request for a name translation that is not within its DNS zone, the DNS server forwards the request to another DNS server within the proper zone for translation. DNS is scalable because hostname resolution is spread across multiple servers.
Computer operating systems have a utility called Nslookup that allows the user to manually query the name servers to resolve a given host name. This utility can also be used to troubleshoot name resolution issues and to verify the current status of the name servers. When the nslookup command is issued, the default DNS server configured for your host is displayed. The name of a host or domain can be entered at the nslookup prompt.
On larger networks, DHCP is preferred for address assignment. Rather than use static addressing for each connection, it is more efficient to have IPv4 addresses assigned automatically using DHCP. DHCP can allocate IP addresses for a configurable period of time, called a lease period. When the lease period expires or the DHCP server gets a DHCPRELEASE message, the address is returned to the DHCP pool for reuse. Users can freely move from location to location and easily re-establish network connections through DHCP.
DHCPv6 provides similar services for IPv6 clients. One important difference is that DHCPv6 does not provide a default gateway address. This can only be obtained dynamically from the Router Advertisement message of the router.
When an IPv4, DHCP-configured device boots up or connects to the network, the client broadcasts a DHCPDISCOVER message to identify any available DHCP servers on the network.
A DHCP server replies with a DHCPOFFER message, which offers a lease to the client. The client sends a DHCPREQUEST message that identifies the explicit server and lease offer that the client is accepting.
Assuming that the IPv4 address requested by the client, or offered by the server, is still available, the server returns a DHCPACK message that acknowledges to the client that the lease has been finalized. If the offer is no longer valid, then the selected server responds with a DHCPNAK message. If a DHCPNAK message is returned, then the selection process must begin again with a new DHCPDISCOVER message being transmitted.
DHCPv6 has a set of messages that is similar to those for DHCPv4. The DHCPv6 messages are SOLICIT, ADVERTISE, INFORMATION REQUEST, and REPLY.
Another module done! What did you learn in this module about IP addressing services? Before taking this module did you think about the DNS protocol defining an automated service that matches resource names with the required numeric network address? Did you know the difference between static addressing and DHCP? I really learned a lot and hope you did too!