Heritage Series ADSL Bridge/ Router | 121 |
DHCP Server Operation |
8.4.1 Network topology
Declarations about network topology include the shared-networkand the subnet declarations. If clients on a subnet are to be assigned addresses dynamically, a range declaration must appear within the subnet declaration. For clients with statically assigned addresses, or for installations where only known clients will be served, each such client must have a host declaration. If parameters are to be applied to a group of declarations which are not related strictly on a per-subnet basis, the group declaration can be used.
For every subnet which will be served, and for every subnet to which the DHCP server is connected, there must be one subnet declaration, which informs DHCP of the addresses present on that subnet. A subnet declaration is required for each subnet even if no addresses will be dynamically allocated on that subnet.
Some installations have physical networks on which more than one IP subnet operates. For example, if there is a site-wide requirement that 8-bit subnet masks be used, but a department with a single physical ethernet network expands to the point where it has more than 254 nodes, it may be necessary to run two 8-bit subnets on the same ethernet until such time as a new physical network can be added. In this case, the subnet declarations for these two networks may be enclosed in a shared-networkdeclaration. Some sites may have departments which have clients on more than one subnet, but it may be desirable to offer those clients a uniform set of parameters which are different than what would be offered to clients from other departments on the same subnet. For clients which will be declared explicitly with host declarations, these declarations can be enclosed in a group declaration along with the parameters which are common to that department. For clients whose addresses will be dynamically assigned, there is currently no way to group parameter assignments other than by network topology. When a client is to be booted, its boot parameters are determined by first consulting that client’s host declaration (if any), then consulting the group declaration (if any) which enclosed that host declaration, then consulting the subnet declaration for the subnet on which the client is booting, then consulting the shared-networkdeclaration (if any) containing that subnet, and finally consulting the top-level parameters which may be specified outside of any declaration. When DHCP tries to find a host declaration for a client, it first looks for a host declaration which has a fixed-addressparameter which matches the subnet or shared network
