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DHCP and NAT

DHCP and NAT. By - Ricardo Sanchez, Ken Wolters and William Hibbard . What is DHCP ?. DHCP stands for "Dynamic Host Configuration Protocol“. What is DHCP's purpose?

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DHCP and NAT

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  1. DHCP and NAT By - Ricardo Sanchez, Ken Wolters and William Hibbard Valencia Community College - Wan Design

  2. What is DHCP ? DHCP stands for "Dynamic Host Configuration Protocol“

  3. What is DHCP's purpose? DHCP's purpose is to enable individual computers on an IP network to extract their configurations from a server (the 'DHCP server') or servers, in particular, servers that have no exact information about the individual computers until they request the information.

  4. The overall purpose of this is to reduce the work necessary to administer a large IP network. The most significant piece of information distributed in this manner is the IP address.

  5. Who Created It? How Was It Created? DHCP was created by the Dynamic Host Configuration Working Group of the Internet Engineering Task Force (IETF; a volunteer organization which defines protocols for use on the Internet).

  6. How Does DHCP Work? When a client needs to start up TCP/IP operations, it broadcasts a request for address information. The DHCP server receives the request, assigns a new address for a specific time period (called a lease period) and sends it to the client together with the other required configuration information

  7. What protocol and port does DHCP use? DHCP runs over UDP, utilizing ports 67 and 68.

  8. What is a DHCP lease? A DHCP lease is the amount of time that the DHCP server grants to the DHCP client permission to use a particular IP address. A typical server allows its administrator to set the lease time.

  9. Can a DHCP server back up another DHCP server? You can have two or more servers handing out leases for different addresses. If each has a dynamic pool accessible to the same clients, then even if one server is down, one of those clients can lease an address from the other server.

  10. Other DHCP Features - • Ability to limit what MAC addresses will be served with dynamic addresses • Server grace period on lease times • Ability to force client's) to get a new address rather than renew • Use of PING (ICMP Echo Request) to check an address prior to dynamically allocating it • Ability to define the pool/pools of addresses that can be allocated dynamically

  11. What Routers use DHCP to configure their IP addresses? The DHCP RFC specifically says that DHCP is not intended for use in configuring routers. The reason is that in maintaining and troubleshooting routers, it is important to know its exact configuration rather than leaving that to be automatically done, and also that you do not want your router's operation to depend upon the working of yet another server.

  12. What Advantages Does DHCP Have Over Manual Configuration Methods? • Manual configuration requires the careful input of a unique IP address, subnet mask, default router address and a Domain Name Server address • Problems can occur when manually setting up your client machines, so if you have the option to set-up your client machines automatically, please do, as it will save you time and a lot of headaches • Unfortunately, we do not live in an ideal world; computers are frequently moved and new systems get added to a network.

  13. Advantages / Disadvantages Cont. • Each computer gets its configuration from a "pool" of available numbers automatically for a specific time period meaning no wasted numbers. When a computer has finished with the address, it is released for another computer to use • Configuration information can be administered from a single point. Major network resource changes (e.g. a router changing address), requires only the DHCP server be updated with the new information, rather than every system.

  14. Can DHCP Provide Support For Mobile Users? • Very simply put, Yes. The benefits of dynamic addressing are especially helpful in mobile computing environments where users frequently change locations

  15. Can a DHCP client update its DNS entry through DHCP? No. There has been some discussion about adding this ability to DHCP. A DNS Server is needed for this service. Which may be possible with a Cisco Router!?!?

  16. Commands to enable DHCP through Cisco Routers • Router(config-dhcp)# network network-number [mask |/prefix-length] Specifies the subnet network number and mask of the DHCP address pool

  17. Enabling the Cisco IOS DHCP Server Feature • Router(config)# service dhcp Enables the Cisco IOS DHCP Server feature on your router. Use the “NO” form of this command to disable the Cisco IOS DHCP Server feature.

  18. Commands Cont. Step – 1 • Router(config)# ip dhcp pool name Creates a name for the a DHCP server address pool and places you in DHCP pool configuration mode—identified by the (config-dhcp)# prompt. Step – 2 Router(config-dhcp)# host address [mask | /prefix-length] Specifies the IP address and subnet mask of the client.

  19. Debugging Command • Router# debug ip dhcp server {events | packets | linkage} Enables debugging on the DHCP server

  20. Excluding IP Addresses • Router(config)# ip dhcp excluded-address low-address [high-address] Specifies the IP addresses that the DHCP server should not assign to DHCP clients.

  21. Whats the Ip-Helper Address Do ? Allows a client on a different segment than the server is located on to be able to receive it’s IP address. Since the Router will not automatically forward the information, the Ip-helper address will do the work.

  22. Ip – Helper Address • Router(Config)# interface (whichever your applying to) • Router(config-if)# ip helper-address 10.1.2.2 • The general rule is to configure the command on the interface closest to the client. The address used in the ip helper-address can be a specific server IP address or it can be the the network address if other DHCP Servers are on the destination network segment. Using the network address allows other servers to be able to respond, also

  23. NAT (Network Address Translation) Network Address Translation allows a single device, such as a router, to act as agent between the Internet (or "public network") and a local (or "private") network.

  24. NAT, as defined by RFC 1631, is the process of swapping one address for another in the IP packet header. In practice, NAT is used to allow hosts that are privately addressed (using RFC 1918 addresses) to access the Internet.

  25. NAT… The most powerful feature of NAT routers is their capability to use port address translation (PAT), which allows multiple inside addresses to map to the same global address. This is sometimes called a "many-to-one" NAT. With PAT, or address overloading, literally hundreds of privately addressed nodes can access the Internet using only one global address.

  26. Reasons for NAT • The shortage of IP addresses • Security • Administration

  27. THE END…..

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