Jump to content


 


Register a free account to unlock additional features at BleepingComputer.com
Welcome to BleepingComputer, a free community where people like yourself come together to discuss and learn how to use their computers. Using the site is easy and fun. As a guest, you can browse and view the various discussions in the forums, but can not create a new topic or reply to an existing one unless you are logged in. Other benefits of registering an account are subscribing to topics and forums, creating a blog, and having no ads shown anywhere on the site.


Click here to Register a free account now! or read our Welcome Guide to learn how to use this site.

Photo

Can DHCP detect used address and skip them?


  • Please log in to reply
6 replies to this topic

#1 mastersmok

mastersmok

  • Members
  • 2 posts
  • OFFLINE
  •  
  • Local time:07:01 AM

Posted 13 April 2015 - 08:42 AM

Greetings,

 

I have the following scenario/situation that I am trying to get clarification on. Let's say you have a smart switch/router. You set it up with a DHCP pool of 192.168.1.20-250. If I set up a device connected to the network with a static address of 192.168.1.25 and then connect 10 additional devices which are all using DHCP. Will the initial device realize there is something already using .25 and "skip" over that address. Or will it assign the .25 to one of the ten devices and just create a conflict. Basically my question is, in this case a cable modem, is it smart enough to see what devices and what address are connected to it and when assigning DHCP address, will it skip over any address that is used by a device using a static address.

Thank you for any insight into the situation.



BC AdBot (Login to Remove)

 


m

#2 Aura

Aura

    Bleepin' Special Ops


  • Malware Response Team
  • 19,200 posts
  • OFFLINE
  •  
  • Gender:Male
  • Location:Quebec, Canada
  • Local time:06:01 AM

Posted 13 April 2015 - 08:44 AM

Hi mastersmok :)

Networking is really not my strong point, however, I believe that if you want the DHCP to skip IP addresses when assigning IPs, you can add a "reservation plage", where you'll enter IP addresses that are already given to devices on the network, so the DHCP won't assign them again. I guess this is just the "beginning" of an answer, I'll let someone more knowledgeable than me in Networking give you a full, complete explanation :)

unite_blue.png
Security Administrator | Sysnative Windows Update Senior Analyst | Malware Hunter | @SecurityAura
My timezone UTC-05:00 (East. Coast). If I didn't reply to you within 48 hours, please send me a PM.


#3 CaveDweller2

CaveDweller2

  • Members
  • 2,629 posts
  • OFFLINE
  •  
  • Gender:Male
  • Local time:07:01 AM

Posted 13 April 2015 - 10:44 AM

If DHCP didn't assign it, then it knows nothing about it. But if you are statically assigning addresses not in the excluded address pool then you are making a mistake in your addressing scheme. 

 

Most people at home are running on the 192.168.x.x/24 scheme which give you 253 addresses for devices. With 0 being the network, 255 being broadcast and either 1 or 254(normally) being the router. So unless you run a server farm from your house or something that is WAY more addresses than you'll need for a long while. So start your addressing at 100 and don't worry about it, which, in my experience, is what Linksys does out of the box.


Hope this helps thumbup.gif

Associate in Applied Science - Network Systems Management - Trident Technical College


#4 mastersmok

mastersmok
  • Topic Starter

  • Members
  • 2 posts
  • OFFLINE
  •  
  • Local time:07:01 AM

Posted 13 April 2015 - 10:59 AM

Alright, thank you. That's precisely what I needed to know, if DHCP didn't assign it, will it know there's still something there? If it doesn't know then I will have to change my available pool accordingly. I was really hoping a device that has a dhcp server on it would be smart enough to see what else is on the network, but I guess not. Regardless, thank you for the answer and help.



#5 Aura

Aura

    Bleepin' Special Ops


  • Malware Response Team
  • 19,200 posts
  • OFFLINE
  •  
  • Gender:Male
  • Location:Quebec, Canada
  • Local time:06:01 AM

Posted 13 April 2015 - 11:02 AM

I guess you were expecting the switch to send a ping in every available IP it have in its range, then see which one replies so it'll know which addresses are taken? Wouldn't work if the device is offline with a static IP (like shut down) :P

unite_blue.png
Security Administrator | Sysnative Windows Update Senior Analyst | Malware Hunter | @SecurityAura
My timezone UTC-05:00 (East. Coast). If I didn't reply to you within 48 hours, please send me a PM.


#6 CaveDweller2

CaveDweller2

  • Members
  • 2,629 posts
  • OFFLINE
  •  
  • Gender:Male
  • Local time:07:01 AM

Posted 13 April 2015 - 11:21 AM

DHCP servers are listening for a broadcast requesting an IP and then they act/respond. So they are passive. So a DHCP server doesn't know nor care if you have an address unless they assigned it or you are requesting one.


Hope this helps thumbup.gif

Associate in Applied Science - Network Systems Management - Trident Technical College


#7 Kilroy

Kilroy

  • BC Advisor
  • 3,284 posts
  • OFFLINE
  •  
  • Gender:Male
  • Location:Launderdale, MN
  • Local time:05:01 AM

Posted 13 April 2015 - 04:44 PM

This is also an issue when the DHCP server goes down but machines that received addresses are still up.  When the DHCP server comes back up it doesn't know what addresses were assigned when it went down and just starts handing them out again.

 

As said if you want something to always get the same address in the DHCP range you create a reservation.  Depending on your DHCP server you can create reservations outside of the DHCP range my Netgear router won't allow it.  Normally you want static devices outside of your DHCP range, but reservations allow you to have them within the DHCP scope.






0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users