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Plan and Implement DHCP on Your Subnet

This article applies to: DNS


Consider the configuration options described below when planning your subnet for DHCP. When you've determined your needs, CIT will assist with set up. Contact CIT by sending email to the hostmaster with your requests and/or questions.

CIT's DHCP Service Can be Customized for Your Environment

  • DHCP connections from hosts to the server may be either:
    1. A known host or MAC (one that is registered with the DHCP server): Your DHCP network can handle known hosts in two ways:
      • If the current and registered subnets match, use the existing DNS static address.
      • If the subnets are different, either refuse to extend an IP address or use the Dynamic Pool to assign a temporary address on this subnet.
    2. An unknown host or MAC (a new system that is from an unregistered department or off-campus site): Your DHCP network can handle unknown hosts in two ways:
      • Refuse to assign a temporary address on this subnet.
      • Use the Registration Pool to assign a temporary address and send the user to the host registration page.
  • DNS subdomain name: Which existing DNS subdomain name is associated with your subnet within the DHCP service? An appropriate subdomain may follow this form: dept.cornell.edu.
  • Lease duration: The default lease duration is:
    • 5 minutes for registration pools
    • 1 hour for dynamic pools
    • 1 hour for DHCP statically assigned addresses
      If you need other options please discuss it with CIT at configuration time.
  • Prefix for DNS registration of the pool addresses: Most DHCP hostnames include dhcp and some portion of the IP address and/or location (location may not be practical on subnets that cross various buildings or in buildings housing several departments). For example:
    • 128.84.123.XXX dhcp-<building>-XXX.dept.cornell.edu
    • 128.84.59 dhcp-bakerlab-59.chem.cornell.edu
    • 128.253.2.110 dhcp-bradfield-110.css.cornell.edu
  • Distributions of the IP addresses for your subnet among statically assigned addresses, dynamic, and registration pools: DHCP can deliver a static DNS registered IP address to a specific system or it can dynamically select an address to be delivered. DHCP selection can be set up in one of three ways:
    • Option 1 (strongly recommended): A large static host space with small dynamic and registration pools. Unknown systems receive a temporary, restricted IP address and the user is directed to a web page to register and obtain a static DHCP IP address. A small dynamic pool for known MAC addresses only should also be included; this will allow a user who is registered on a different subnet within CIT's DHCP server to connect to your net and receive a temporary address from this dynamic pool without having to re-register. The dynamic pool can be on-campus restricted or fully Internet enabled.
    • Option 2: Same as Option 1, but with no dynamic pool. This option further restricts the subnet to its own users.
    • Option 3 (Best used for public nets or lab spaces): One large dynamic pool with a small floating registration pool. This would allow anyone to register and become known to the DHCP server without being assigned a static IP address.

Examples

Example 1: Department Subnet DHCP on the 128.253.230 subnet for department ONR

Assign To provide
128.253.230.11-244 Static IP addresses. Allow users to choose their own host names.
10.253.230.11-244 Static IP addresses assigned by the netadmin.
128.253.230.245-254 Dynamic pool. Allow addresses for known hosts for Cornell faculty and staff who plug into the subnet for lectures or discussions.
10.253.230.245-254 Registration pool. Set aside 10 addresses for registration of unknown hosts.

DHCP Domain

DHCP DNS Domain Name onr.cornell.edu

Dynamic Pool Information (for known hosts)

Starting IP address 128.253.230.245
Ending IP address 128.253.230.254
DNS base name dhcp-bakerlab-59-

Registration Pool Information (for unknown hosts)

Starting IP address 10.253.230.245
Ending IP address 10.253.230.254
DNS base name reg-bakerlab-59-
Allow user to choose host name yes
User registration will be static

Example 2: Student lounge or class room, registered

DHCP for a student area subnet in department BIB. Dynamic address range 128.253.210.11-60. Anyone can connect to this. Students with computers on the wired network will get a number without registration. Students from off-campus will have to register once to become "known" to the DHCP server. No static IP addresses are assigned. Lease for 10 minutes to free up addresses after students leave.

DHCP Domain

DHCP DNS Domain Name bib.cornell.edu

Dynamic Pool Information (for known hosts)

Starting IP address 128.253.210.11
Ending IP address 128.253.210.60
DNS base name dhcp-bib221
Lease Timeout (seconds) 600

Registration Pool Information

Starting IP address 10.253.210.11
Ending IP address 10.253.210.60
DNS base name reg-bib221
Allow user to choose host name no
User registration will be floating

Clearing a Block of Addresses for Use in DHCP

Systems that are already registered with DNS and that also fall into an IP address range that will be used for a DHCP pool need to be removed. This does not affect hosts outside those ranges, which can be left alone, or you can add the MAC addresses to those hosts to create a DHCP static assignment. You can examine your registered host list on the DNS database web page.

About this Article

Last updated: 

Monday, December 5, 2016 - 3:50pm

Audience: 

IT Professionals

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