Hi, not too sure what you need but I will give it a go,
I have attached an image which shows three different possible topologies.
1)
This is your network at the moment, the pink spot shows your CAT-5 network connection, your DHCP server is in blue and I have drawn 3 PC's on your LAN through a switch. IP details below :
Default Gateway : 192.168.100.1
Subnet Mask : 255.255.255.0
PC1 : 192.168.100.10
PC2 : 192.168.100.11
PC3 : 192.168.100.12 <<<< Your PC
2)
This is as above but you have unplugged your PC and plugged in a switch then plugged your PC and others into the switch.
The DHCP server will then assign additional addresses to the PC's you have just added (This is the cleanest way to do it), IP details below :
Default Gateway : 192.168.100.1
Subnet Mask : 255.255.255.0
PC1 : 192.168.100.10
PC2 : 192.168.100.11
PC3 : 192.168.100.12 <<<< Your PC
PC4 : 192.168.100.13
PC5 : 192.168.100.14
3)
Double Natting (Satan in networking terms),
This is as above but instead of adding a switch you add a router, this will be the solution if you need to use the 192.168.100.12 address for ALL of the additional PC's. This would also be the required solution if your DHCP server has a MAC filter or static DHCP assignments (and you don't administer your DHCP server).
For this you would need to add a router (Wired or wireless) and set the MAC of the WAN port to the MAC of your PC. The router would then be assigned "your" IP address and you would need to setup NAT on your new mini LAN. IP info below :
Main LAN
Default Gateway : 192.168.100.1
Subnet Mask : 255.255.255.0
PC1 : 192.168.100.10
PC2 : 192.168.100.11
Router : 192.168.100.12
Mini LAN
Default Gateway : 10.10.10.1
Subnet Mask : 255.255.255.0
PC3 : 10.10.10.2
PC4 : 10.10.10.3
PC5 : 10.10.10.4
PC6 : 10.10.10.5
This is a very "dirty" solution as apposed to "2" but if you don't administer your LAN and you need to use just your IP address to allow asscess to the additional PC's this would be the only viable solution.
Hope this helps, If ive gone off in the wrong direction please post more info re your needs,