Does an IP from DHCP server/router always overwrite an already existing fixed local IP ?

  • Thread starter Thread starter Steven Prasil
  • Start date Start date
S

Steven Prasil

Assume I assign a fixed IP to my local computer. My local computer in turn is
connected through in-house LAN to a router which acts as well as a DHCP server.
Both (local computer and router) are within the same subnet: 192.168.1.50 and 192.168.1.1)

Does the DHCP server in the router recognize the already existing fixed IP and
avoids a new assignment (of a dynamic IP) ?

Or does a DHCP server ALWAYS generate new IPs and force his clients to accept the new IP ?

Steven
 
Assume I assign a fixed IP to my local computer. My local computer in turn is
connected through in-house LAN to a router which acts as well as a DHCP server.
Both (local computer and router) are within the same subnet: 192.168.1.50
and 192.168.1.1)

Does the DHCP server in the router recognize the already existing fixed IP and
avoids a new assignment (of a dynamic IP) ?
Click to expand...

All of the routers I've dealt with were happy to support the static IP address
assigned to a PC (as long as it's in the range for the local subnet).
Or does a DHCP server ALWAYS generate new IPs and force his clients to
accept the new IP ?
Click to expand...

No. If you assign a static IP address to your machine, it won't even
initiate DHCP. The router can't give your machine an IP address unless
your machine asks for one (with DHCP). Since your machine has a static
IP address, it's not going to ASK for another one.

Patrick
========= For LAN/WAN Protocol Analysis, check out PacketView Pro! =========
Patrick Klos Email: (e-mail address removed)
Klos Technologies, Inc. Web: http://www.klos.com/
==================== http://www.loving-long-island.com/ ====================
 
No. If you assign a static IP address to your machine, it won't even
initiate DHCP. The router can't give your machine an IP address unless
your machine asks for one (with DHCP). Since your machine has a static
IP address, it's not going to ASK for another one.

Patrick
========= For LAN/WAN Protocol Analysis, check out PacketView Pro! =========
Patrick Klos Email: (e-mail address removed)
Klos Technologies, Inc. Web: http://www.klos.com/
==================== http://www.loving-long-island.com/ ====================
Click to expand...

But the router will assign the same address via DHCP to another computer on
the LAN, thereby creating an address conflict. In a mixed static and DHCP
network, the DHCP scope should be defined and static addresses should be
assigned from outside the DHCP scope.
 
But the router will assign the same address via DHCP to another computer on
the LAN, thereby creating an address conflict. In a mixed static and DHCP
network, the DHCP scope should be defined and static addresses should be
assigned from outside the DHCP scope.
Click to expand...

Not necessarily. Most routers will send out an ARP for an IP address
they're thinking of assigning to a DHCP client. If they get a reply to
the ARP, they know that IP is taken and move on to another IP address to
try. This is also how the network can stay intact even if the router is
rebooted and doesn't know who it gave which IP address to.

========= For LAN/WAN Protocol Analysis, check out PacketView Pro! =========
Patrick Klos Email: (e-mail address removed)
Klos Technologies, Inc. Web: http://www.klos.com/
==================== http://www.loving-long-island.com/ ====================
 
"most routers"? Certainly not a single soho router, which is what we are
discussing here, does arp for dhcp.

The router, after a reboot, looks at the lease times in dhcp, which is why
"network can stay intact even if the router is rebooted and doesn't know who
it gave which IP address to" because IT DOES know.

BobC is correct that if you do not exclude your staticly assigned ip address
there is a strong chance that you will end up with a ip conflict due to the
dhcp assignment of your static ip address.
 
Not necessarily. Most routers will send out an ARP for an IP address
they're thinking of assigning to a DHCP client. If they get a reply to
the ARP, they know that IP is taken and move on to another IP address to
try. This is also how the network can stay intact even if the router is
rebooted and doesn't know who it gave which IP address to.
Click to expand...

This is only true if the computer with the static address aquires the IP
prior to the dhcp server assigning it to another computer. If the computer
with the static address is offline and the dhcp server is unaware of that
computer, then it can assign the address to another computer if it is within
it's scope.

My suggestion, do it right the first time and either reserve the address or
set the address outside the servers scope. Why would you set yourself up for
an unexpected surprise maybe 6 months down the road when you have completely
forgotten what you did.

Imagine the static addy was assigned to a server and there is a power
failure friday night. Monday morning somebody starts up his computer prior
to the server startup and is assign that addy. Later there are calls that
the server is down, you sit back and ping the servers addy and you see it up
and running, you go physically to the server and see its on, you restart,
ping is still sucessful but still does not work. You look closer and
discover the static ip failed because it is in use, now you have to find out
who's got the addy to correct the situation

James
 
Or if the stacically assigned computer just happens to be offline when the
arp is sent. Some (not all or even most) SOHO routers will attempt to
resolve IP address conflicts before assigning an address. But even if they
do, many situations will still lead to address confllicts. The ONLY safe
approach is to assign statics outside of the DHCP scope.

....kurt
 
Well a dhcp server that allows addry reservations based on MAC addys also
works but I think everyone should agree with using an addy outside of the
server's scope as it is probably the best way to avoid problems down the
road.

James
 
Yes, because I THOUGHT that's what we were discussing: Windows 2000
networking, not Linksys networking.

Let's say I have a network at 131.107.2.xxx. I assign my server .200
and my laptop .213. All I have to do is go into the DHCP Console and
put those in as addresses to not assign.

But if we are OT and talking about a SOHO router, yes, assign it out of
scope of the DHCP (but not outside the subnet of the router, e.g. the
router might do from .1 through .57, so do .58+ for your statics).
 
Back
Top