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DHCP Tutorial. Technology every Admin needs to know. Networking & TCP/IP Tutorial. TCP/IP Explained.
DHCP Stands for Dynamic Host Configuration Protocol, which is used to dynamically assign IP addresses to client computers.
This video will be in three parts:
First I go over all the basic concepts of DHCP
Second I do a live demonstration setting up a simple DHCP server.
And finally we go down to the packet level of DHCP in Wireshark. And if you understand the protocol down to the packet level, then you really understand that protocol.
Let’s start by remembering that each device, or HOST, on a TCP/IP network requires a unique IP address. Recall that IP addresses are 32 bits long and generally represented in dotted decimal notation of four octets, such as 192.168.1.27.
There are two possible ways that host can obtain its unique IP address: either assigned Statically (ie manually) by an admin or dynamically (ie automatically) by DHCP. In the old days you had to statically assign an IP address to each and every device in the network. This required a great deal of time and effort, and was subject to errors such as accidentally assigning to hosts to the same IP address. So nowadays DHCP is almost always used. In fact DHCP is so common that it is built-in to most Soho wireless routers.
DHCP is a server service ie a service that will be performed by a server, thus it resides on layer 7 of the OSI reference model, which is the application layer.
For all you newbies, remember that DNS is a separate service from DHCP, even though they can sometimes talk to each other.
When we say that DHCP provides IP addresses to hosts, it actually provides 4 parameters to the host. These are:
1. IP address,
2. subnet mask
3. default gateway
4. primary and secondary DNS servers
There are three other terms used in DHCP that we need to understand. These are Scope, Leases and Reservations
SCOPE: The IP addresses that are assigned dynamically are pulled from a scope on the server. Scope is range of IP addresses that DHCP server is allowed to give out
LEASES: Leases assign the IP address to the client for a specific amount of time configurable by the admin. This helps make sure that you don’t run out of IP addresses, insuring a host doesn’t hold onto an IP address indefinitely after the computer is not on the network anymore. A client generally tries to renew its lease at 50% of the lease time, and when the lease is renewed it will generally keep the same IP.
RESERVATIONS can be assigned to specific servers from within the scope, and will never be given out to other computers on the network. This allows the server to maintain the same IP address indefinitely, which is generally desirable.
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