If u define the same vlan for phones and PC they will in the same broadcast domain hence they will get the IP address from the same subnet.

If u r using a seprate vlan for voice then u can define a seprate subnet for phones.

The best practice is to have seprate vlans for pc and phones.

U r going to define the switch port in data vlan and voice vlan.The switch is going to sense the presence of the voice device and place it in the voice vlan defined on the port so as the vlan=broadcast domain it is going to get the ip from the specified subnet.

Thanks

Mahmood

This is how our ports are configured....

interface GigabitEthernet1/4

switchport access vlan 124

switchport mode access

switchport voice vlan 224

qos trust cos

qos trust device cisco-phone

auto qos voip cisco-phone

tx-queue 3

bandwidth percent 33

priority high

shape percent 33

spanning-tree portfast

service-policy output autoqos-voip-policy

Here is how our VLAN's are configured

interface Vlan124

description 24th Floor Workstations VLAN

ip address 192.168.124.1 255.255.255.0

ip helper-address 192.168.2.43

no ip redirects

ip pim sparse-mode

interface Vlan224

description VoIP 24th floor

ip address 192.168.224.1 255.255.255.0

ip helper-address 192.168.2.43

Hope this helps.

Burleyman

Its good to have separate subnets for data & voice vlan. When you specify different vlans on switchport for voice & data then voip phone behaves as if the port was trunk & sends voice pkts as tagged. On the other hand, pc's in data vlan send its pkts untagged as in case of native vlan on trunk port.

Its preferred to keep the port as access port & configure it as in the above post.