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Understanding the Basic Operations of DHCP
October 23, 2013 | By Netmanias (tech@netmanias.com)
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Comments (21)
hamed 2015-05-29 17:52:57

 ijust can say thanks,very thanks,it was complete article

Indrasen 2015-09-05 02:36:48

Excellent article. Clearly put. Great work. Thank you.

Veeresh 2017-11-27 19:42:04

Nice article to understand.

ammi 2019-01-08 23:39:00

very good article. one place to viist to understand DHCP in detail...

netmaniac 2019-01-21 17:08:46

An amazing article, really well written. Keep up the good work. I recommend this to all network engineer. This is the only article series where I found everything I need in one place.

Studente 2019-02-16 17:37:52

Very clear, nice article

Prakash 2019-11-26 01:46:05

Very good informative article. 

Shrivan Raj 2020-01-03 19:31:00


Article is very clear. But I have two doubts.

Doubt 1:
Asssume that client got the ipc at 1pm with lease time of 1hr. So, Till 2pm we can use this ip. now the client will request again at 1.30pm for renewal. Now again assume that we got one hr in this renewal process. Here comes my question is, already we have acess for the time 1.30pm to 2pm. Now after renewal(1 hr). So now our ip is allowed till 3pm or time reset from 1.30pm to 2.30 pm ?? 

If our ip is allowed only till 2.30 then looks like we got only half an hour extra time(from 2pm to 2.30pm). 

Doubt 2:
In the DHCP paylod, we have client Identifer field(option 51) & also Client Hardware address. For DHCP Request we are filling client MAC in both place. What is the specific purpose. 

If no specific reason, then wy we need to fill twice??

Could you clarify my doubt?

TA 2020-11-04 22:45:24

On D-1: I think it is obvious that at 1:30 you will renew for 1 hour until 2:30. Renewal starts at 1:30 and not at 1:59 because operation may fail and need retry and we don't need network intterruption even for 1 second. In other words, the real lease time is half hour and DHCP server gives twice time the chance to get new lease.

Hacker 2020-02-13 02:23:15

Thank you very much for the explanation, You rock!

otmann 2020-03-03 02:29:44

very good description

belajar 2020-03-20 17:17:50

Hi , Just curious , you are using router ip address --> is this as a gateway ? because in a header i saw the gateway ip address is thanks

robert down 2020-04-01 07:59:21

Explained perfectly. I've read about this on wikipedia formerly but didn't understand correctly but here understood completely

Sa 2020-05-15 10:32:19

isn't the DHCP offer and Ack unicast ???

WolFie007 2020-10-13 20:55:03

DHCP already have - siaddr and giaddr fields right; but why do they use options field to provide router IP and DHCP IP when they already have those fields? 

Moreover, I have captured the packets in Wireshark; could see the same thing; also observed that it lacks options other than 53 and 54 (here in my case my DHCP server= gw=dns); so is there any field which when set to 1 or 0 indicates that everything is same?

Balaji 2021-04-13 23:47:13

Very good explanation. Thank you for your detailed descriptions.

Gouthami 2021-08-27 03:41:35

Easy to understand the background on how DHCP works

Dieter Busch 2021-09-22 20:15:20

Just working on a question regarding netmasks and DHCP. Should a client in renew state recognise and deploy a modified netmask in the ACK message? We are facing the question of how to extend networks "on the fly" and would like to avoid restarting the complete process.

holla 2021-10-08 21:34:16

in the last step, internet access with SIP= is impossible, because it is just a private IP address. NATTing is required.

Benno 2021-10-22 15:05:11

Wow... great article. What I miss in this article, is the following case: 
If the unicast DHCP RENEWAL process fails because of a unicast connectivity issue, then after T2 (87.5% of the lease time) the client it will do the renewal or rebinding with broadcast until the lease expires. This process is called rebinding. (https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-2000-server/cc958935(v=technet.10)?redirectedfrom=MSDN)

Danny.V 2023-12-06 03:01:52

So good, cheers bro!

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