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Why should I assign static IP address on my DHCP server?

gplana
gplana asked
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Hi.

I have installed isc-dhcp-server on a Debian. I'm making some experiments with this as I'm a little bit newbie on network administration.

I have read that you should assign static IP to any ethernet interface from where you want to serve IP addresses from your DHCP, but I don't really understand why.

I mean, why my DHCP server can't assign an IP address to "myself" ? All I need is to define a rank of IP addresses on dhcpd.conf, isn't it?

Thank you.
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I think what you read is that your DHCP server itself should have a static IP address.  That is correct, and in my opinion all servers should have a static IP address.  

A lot of services (BIND, ISC-DHCP, etc..) bind to a certain IP address.  If they were receiving DHCP addresses it would make it very hard to administer.
Don JohnstonInstructor
Top Expert 2015

Commented:
One reason is that when a host tries to renew it's address, it will do so to the IP address of the server.  If the server address changes, then it can't renew the address.
I understand that the DHCP server need to have a FIXED address, but why couldn't it be assigned by the DHCP server itself?

I mean, DHCP server can assign fixed address to hosts, according to its MAC address...
Don JohnstonInstructor
Top Expert 2015

Commented:
It can, but what's the gain?  

The primary function of DHCP is to ease in the deployment of hosts so you don't have to manually assign IP addresses.  We're talking one host here.  So there's no real gain to having this server get a dynamic address.
Distinguished Expert 2018
Commented:
Maybe because network  card, when asking for it's address sends DHCP request to other network devices (not to OS).
Network card will be configured by OS in case that network card don't receive any offer from some DHCP server.
:)
Aside from -  renew address from the same DHCP server.

Also DHCP server can be responsible for assigning multiple DHCP scopes, not just one. What ip address should then get network card, exclusions, reservations - it can get really complicated if you don't assign static ip address.
bbaoIT Consultant
Commented:
> I mean, why my DHCP server can't assign an IP address to "myself"

you are asking a really great question as it could be challenging most experts. :))

i guess you have learnt some basic concepts how DHCP works for both DHCP server and DHCP client sides. briefly speaking, both sides need to 'talk' to each other back and forth in order to work out an IP to be assigned to the DHCP client. how can they talk to each other?

be aware that during the negotiation process, the DHCP client has NOT obtained an IP, so the both sides can't talk to each other at IP layer. instead, they talk at Link Layer, which uses MAC address to locate each other. theoretically, any Ethernet device has an unique MAC address, that's the ID for the devices to differentiate themselves individually.

okay, if, just IF, the DHCP server is to assign an IP to itself, then how does the server talk to the client over the SAME MAC address? can you know who talks to who and who serves who? apparently, it doesn't make sense and won't work that way. :-)

moreover, if a DHCP server didn't have a static IP, as an administrator how could you know where your DHCP server is located? where to manage it? that's NOT practical in reality and could be a nightmare if it was true. :-)

does it make sense now?
Thank you both.
I think the clue is the IP broadcast address (255.255.255.255) where DHCP questions goes to. I didn't realiza that this means that request is going to the network, but not to myself.
bbaoIT Consultant

Commented:
yes, any broadcast packet from self to broadcast address will be ignored.

but, again, the story starts at Link Layer (Layer 2) and there is NO IP address (Layer 3) at that moment. it is all about MAC address, its broadcast address is FF:FF:FF:FF:FF:FF (48 bits) instead of FF.FF.FF.FF (255.255.255.255 or 32 bits).
I have read that when no IP is assigned, DHCP client uses 0.0.0.0 as IP address, and 255.255.255.255 (=broadcast) as destination.