Friday, October 8, 2010

DHCP PROCESS Summary

Why do we need to know the DHCP lease Process? You are doing troubleshooting why client is not getting IP address from DHCP server and you forgot to see the ports 67 and 68 was blocked. In any case knowing your stuff is going to make you smart and stronger. I recommend keep memorizing below simple process just in case if you ever been any of the situations I have speculated.
In TCP/IP little world a device cannot communicate to any other device unless it has an IP address. Now think about, XP client who has not have any IP address yet, able to locate the DHCP server and ask for an IP address. IN logic world you should say, hey wait a second how come client even can talk to DHCP server, client does not have any IP Address yet.

• At the time of the lease request, the client doesn't know what its IP address is, nor does it know the IP address of the server. To work around this, below how client is able to talk to a DHCP server
• Client uses 0.0.0.0 as its address and assumes 255.255.255.255 for the server's address.
• DHCP discover message on UDP port 68 and destination port 67.
• The discover message contains the hardware MAC address and NetBIOS name of the client.
• Once the first discover message is sent, the client waits 1 second for an offer. If no DHCP server responds within that time, the client repeats its request four more times at 2-, 4-, 8-, and 16 second intervals, if the client still doesn't get a response, it will revert to Automatic Private IP Addressing (APIPA) and Continue to broadcast discover messages every 5 minutes until it gets an answer. With APIPA, (169.254.X.Y) The Windows client will automatically pick what it thinks is an unused address.

DHCP lease is 4 way Process as listed below.
• DHCP – discovery (The discover message contains the hardware MAC address and NetBIOS name of the client.)
• DHCP - Lease offer
• DHCP - lease request
• DHCP- Lease acknowledgment
To keep memorize the process use simple map below
• DD ( DHCP Discovery)
• LO (Lease Offer)
• LR (Lease Request)
• LA (Lease Acknowledgment)

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