13 - DHCP Service.ppt
- 1. DHCP Service
- 2. Overview • The Dynamic Host Configuration Protocol (DHCP) is a set of rules used by a communications device such as a computer, router or network adapter to allow the device to request and obtain an IP address from a server which has a list of addresses available for assignment. • IP addresses and other parameters such as the default gateway, subnet mask, and IP addresses of DNS servers can be assigned by the DHCP server. • The DHCP server ensures that all IP addresses are unique. • DHCP functionally became a successor to the older BOOTP (Bootstrap protocol), whose leases were given for infinite time and does not support options (DNS, Router, addresses).
- 3. DHCP Process • IP address assignment occurs when the DHCP client boots up or regains connectivity to the network. • The DHCP client sends out a query requesting a response from a DHCP server. • The DHCP server then replies to the client with its assigned IP address, subnet mask, DNS server and default gateway information. • The assigned IP address generally expires after a predetermined period of time, at which point the DHCP client and server renegotiate a new IP address from the server's predefined pool of addresses. • Office networks should use DHCP, in particular when workers make extensive use of laptops which link directly to the network occasionally.
- 4. DHCP operations • DHCP operations fall into four basic phases. These phases are – IP lease request, – IP lease offer, – IP lease selection, and – IP lease acknowledgement.
- 5. DHCP discovery • The client broadcasts on the local physical subnet to find available servers. • This client-implementation creates a UDP packet with the broadcast destination of 255.255.255.255 or subnet broadcast address.
- 6. DHCP offers • When a DHCP server receives an IP lease request from a client, it extends an IP lease offer. • This is done by reserving an IP address for the client and broadcasting a DHCPOFFER message across the network. • This message contains the client's MAC address, followed by the IP address that the server is offering, the subnet mask, the lease duration, and the IP address of the DHCP server making the offer.
- 7. DHCP requests • Whenever a computer comes on line, it checks to see if it currently has an IP address leased. If it does not, it requests a lease from a DHCP server. • Because the client computer does not know the address of a DHCP server, it uses 0.0.0.0 as its own IP address and 255.255.255.255 as the destination address. Doing so allows the client to broadcast a DHCPDISCOVER message across the network. • Such a message consists of the client computer's Media Access Control (MAC) address (the hardware address built into the network card) and its NetBIOS name.
- 8. DHCP acknowledgement • When the DHCP server receives the DHCPREQUEST message from the client, it initiates the final phase of the configuration process. • This acknowledgement phase involves sending a DHCPACK packet to the client. This packet includes the lease duration and any other configuration information that the client might have requested. • At this point, the TCP/IP configuration process is complete.
- 9. DHCP terminology • Scope - A scope is the full consecutive range of possible IP addresses for a network. Scopes typically define a single physical subnet on your network to which DHCP services are offered. • Superscope - A superscope is an administrative grouping of scopes that can be used to support multiple logical IP subnets on the same physical subnet. Superscopes only contain a list of member scopes or child scopes that can be activated together. Superscopes are not used to configure other details about scope usage. For configuring most properties used within a superscope, you need to configure member scope properties individually.
- 10. • Exclusion range - is a limited sequence of IP addresses within a scope, excluded from DHCP service offerings. Exclusion ranges assure that any addresses in these ranges are not offered by the server to DHCP clients in the network. • Address pool - After you define a DHCP scope and apply exclusion ranges, the remaining addresses form the available address pool within the scope. Pooled addresses are eligible for dynamic assignment by the server to DHCP clients on your network. • Lease - is a length of time that a DHCP server specifies, during which a client computer can use an assigned IP address. When a lease is made to a client, the lease is active. Before the lease expires, the client typically needs to renew its address lease assignment with the server. A lease becomes inactive when it expires or is deleted at the server.
- 11. • Reservation -You use a reservation to create a permanent address lease assignment by the DHCP server. Reservations assure that a specified hardware device on the subnet can always use the same IP address. This is done by mapping the MAC address to an IP. • Option types - are other client configuration parameters a DHCP server can assign when serving leases to DHCP clients. For example, some commonly used options include IP addresses for default gateways (routers) and DNS servers. Typically, these option types are enabled and configured for each scope.
- 12. Exercise 1. In which range should a server’s IP lie? reserved, excluded or address pool range? 2. A manager wants to have the same IP whenever her PC starts. In which range would her IP lie? 3. There are 10 PCs in a lab used by students. In which range would their IPs lie?