VLAN 'IP' / 'Default Gateway' Query

[blakey2]

Trying to set up vlans.

Currently have the setup:

                                            INTERNET
                                                |                     
                                                |                     
                                                |                     
                      +====+                   WAN
                      |    |              +============+
   (Company2 LAN)     |HP  |  172.16.8.138|    F/W     |10.20.30.40
   172.16.8/20--------|2824|--------------|            |DMZ  --------- (DMZ NET 10.20.30.0/24)
                      |    |              +============+
                      |    |                   LAN1
                      |    |             192.168.144.138
                      +====+                    |
                                                |
                                                |
                                                |
                                      +=================+
                                      |                 |
                                      | HP 2626 Switch  |
                                      +=================+
                                                |
                                                |
                                        (Company1 LAN) 192.168.144.0/24

I want to implement this setup:
ie. Remove router interface, implement vlans to segment networks/subnets and still share internet connection.

                              INTERNET
                                 |                     
                                 |                     
                                WAN
                           +============+
                           |    F/W     |10.20.30.40/24
                           |            |DMZ --------------(DMZ NET 10.20.30.40/24)
                           +============+
                                LAN
                                 |???.???.???.???
                                 |
                                 |
                       +=================+
                       |                 |
                       | HP 2826 Switch  |
                       +=================+
                                 |
                                 |
                                 |
                       +=================+
                       |                 |
                       | HP 2626 Switch  |
                       +=================+
                          |           |
                          |           |
                          |           |
                          |           |
                        vlan2       Vlan3
                          |           |
                          |           |
              192.168.144.0/24      172.16.8.0/20

At this stage i'm unsure about the IP addressing in particular the IP addresses of the switches and the IP address of the default gateway?

I think I should be using a third subnet to address the LAN interface of the router (eg 172.16.12/20 -- '172.16.12.138')??
- I should also use this subnet for the switch IP addresses for VLAN1 (DEFAULT_VLAN)??


Then I have DHCP to contend with. Company1 and Company2 both have completely independant windows server 'forests' which contain their own DHCP servers.
So long as I configure the switch ports correctly, this should be ok (ie untagged VLAN2 or untagged VLAN3), they should get their own IP address leases as they currently do.

To enable internet access though I need to set/update the default gateway to the IP address of the LAN interface of the f/w - ie 172.16.8.12.138 ???
OR should this be the IP address that I set for the VLAN?



I hope this makes some sense??

Cheers - blakey2