Lease Time Guidelines

The default DHCP lease time is 1 day, which is well suited for nets with low turnover (i.e. the population of client machines is fairly stable and doesn’t change much).

Network administrators may choose to configure a different lease time on individual IPv4 DHCP Ranges (Dynamic Pools), subject to the following guidelines which are designed to keep overall load on the DHCP servers from growing unsustainably:

Lease time	Guideline
1 day or longer	Recommended for most networks
between 4 hours and 1 day	No problem, use freely where desired for IPv4 nets with higher host turnover
between 1 hour and 4 hours	Acceptable, but please exercise good judgment and use only where necessary
less than 1 hour	Requires approval from service manager

Because IPv6 subnets are so spacious, there is generally no reason to reduce the DHCPv6 lease time parameters; IPv6 nets with higher host turnover which require Stateful autoconfiguration can simply be configured with larger DHCP Ranges in order to avoid running out of leases.

Range Size Guidelines

IPv4 DHCP Ranges (Dynamic Pools) should be sized so that the number of unassigned IP addresses in the Range (after all desired clients have obtained their leases, and not counting any addresses within Exclusion Ranges) will be at least 1-5% of the total, with a minimum of at least 2 unassigned IP addresses (even for very small Ranges) in order to properly utilize the redundancy provided by DHCP Failover.

Total Addresses in Range	Minimum Unassigned Addresses
3-40	at least 2 addresses
41-200	at least 2-10 addresses (1-5% of total)
201+	at least 1-5% of total

Note: Email Alert thresholds should also be adjusted accordingly (to higher than 95%) if the expected number of unassigned IP addresses is less than 5% of the total.

Service Defaults

The following DHCP settings serve as defaults; they are configured at the top level of the Grid hierarchy and automatically inherited by all Networks. An individual Network, DHCP Range, DHCP Fixed Address, or DHCP-enabled Host can override one of these defaults by specifying its own value for the setting.

Client options

For DHCPv4:

code	name	value
6	`domain-name-servers`	130.126.2.131
42	`ntp-servers`	130.126.24.24, 130.126.24.44, 130.126.24.53
252	`wpad-url`	” ” (a blank space)

Proactively distributing a blank space for option 252 forestalls the Windows client behavior of continually sending DHCPINFORM requests to the DHCP server in an attempt to “automatically detect proxy settings”. This option value can of course be overridden with a real WPAD URL on individual subnets where automatic proxy configuration is desired.

For DHCPv6:

code	name	value
23	`name-servers`	2620:0:e00:a::1

Service settings