Why sub interface down vlan 100

[smcisco]

I am practice for inter vlan routing when i create sub interface 100 and create pool for DHCP does not provide IP to client.

Please see the aatched figure

[Marvin Rhoads]

For a switch's VLAN interface(s) to be up, you must have a port assigned to that VLAN (or a trunk that includes the VLAN) in UP state. You also then need to "no shut" the SVI (Switch VLAN Interface).

Please see this thread over on Cisco Learning Network for more detail.