How DHCP Works
(a) Dynamic Host Configuration Protocol (DHCP)
1- The dynamic host configuration protocol (DHCP) enables computers to receive TCP/IP configuration settings automatically.
2- A DHCP server computer can configure a DHCP client with an IP address and a subnet mask.
3- The DHCP client can also receive other settings from DHCP, such as IP addresses for the default gateway, DNS servers, and WINS server.
4- DHCP is a protocol used to automatically assign TCP/IP configuration parameters to computers. A DHCP server can supply a DHCP client with a number of TCP/IP settings, such as an IP address, subnet mask, and DNS server.
5- If the DHCP server is assigning the IP addresses, only the DHCP server must be configured with IP address information. The only parameter you need to configure on the client end is an option for the client to receive IP address information from the server. The rest of the configuration takes place from the server side manually.
(b) DHCP DISCOVER
1- The DHCP client initiates the process by broadcasting a datagram destined for UDP port 68 (used by BOOTP and DHCP servers).
2- This first datagram is known as a DHCP Discover message, which is a request to any DHCP server that receives the datagram for configuration information.
3- The DHCP discover datagram contains many fields, but the one that is most important contains the physical address of the DHCP client.
(c) DHCP OFFER
1- A DHCP server configured to lease addresses for the network on which the client computer resides constructs a response datagram known as a DHCP offer and sends it via broadcast to the computer that issued the DHCP discover.
2- This broadcast is sent to UDP port 67 and contains the physical address of the DHCP client.
3- Also contained in the DHCP offer are the physical and IP addresses of the DHCP server, as well as the values for the IP address and subnet mask that are being offered to the DHCP client.
(d) DHCP REQUEST
1- The client selects an offer and constructs and broadcasts a DHCP request datagram.
2- The DHCP request datagram contains the IP address of the server that issued the offer and the physical address of the DHCP client. The DHCP request performs two basic tasks. First it tells the selected DHCP server that the client requests it to assign the DHCP client an IP address (and other configuration settings). Second, it notifies all other DHCP servers with outstanding offers that their offers were not accepted.
(e) DHCP ACK When the DHCP server from which the offer was selected receives the DHCP request datagram, it constructs the final datagram of the lease process. This datagram is known as a DHCP ack (short for acknowledgement).
The DHCP ack includes an IP address and subnet mask for the DHCP client. Optionally, the DHCP client is often also configured with IP addresses for the default gateway, several DNS servers, and possibly one or two WINS servers. In addition to IP addresses, the DHCP client can receive other configuration information such as a NetBIOS node type, which can change the order of NetBIOS name resolution.
3. DNS(Domain Name System)
1- DNS is a system used for translating domain names into IP addresses in a TCP/IP based network. The process of resolving a domain name to IP address is called name resolution. The domain name space is a tree-like structure representing all the domains that make up the name space for the Internet. The root domain is at the top of the tree.
2- Defines a hierarchical namespace where each level of the namespace is separated by a "."
3- Provides resolution of names to IP addresses and resolution of IP addresses to names.
4- Capabilities vary by operating system and/or DNS server software version.
5- DNS provides a hierarchical structure. Called domain namespace, for managing the DNS database. The hierarchical structure includes:
Root domain Situated at the top of the domain structure and it is represented by a period (.)
Top-level domain Situated below the root domain and used to represent the type of organization. Following are the top –level domains available.
DNS NAME SPACE
4. QUERY AND LOOKUP TYPES
| Query Types | |
| Iterative query | The DNS server returns an answer to the query or a pointer to other DNS servers |
| Recursive query | The DNS server returns a complete answer to the query, not a pointer to another DNS server |
| Lookup Types | |
| Forward lookup | Requests name-to-address resolution |
| Reverse lookup | Requests address-to-name resolution |
5. DNS Zone Types: Forward and Reverse Lookup
6. RECURSIVE QUERIES
*A query made from a client to a DNS server in which the server assumes the full *workload
*DNS server returns either a complete answer or negative
*Issued by -
*Client computers
*DNS servers configured to use forwarder(s)
7. ITERATIVE QUERIES
1- Receiving server may return an answer, a negative response, or a referral to other DNS server(s).
2- Typically issued by DNS servers not configured to use forwarders for resolution of queries.
3- "Walk" the DNS tree.
4- "Give me an answer or refer me to somebody else who can help me obtain resolution."
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