Need to know that Out of whole network, only one PC from x1.y1.z1.w1 internal network needs to go to internet? Or you already have other internel network already going on internet but just need to add one more PC from different network for internat access?
Looks to me that you have multiple public ip address which can be used for NAT/PAT to allow internal netwrok to access the internet..right?
If yes, then you can configure like this
ip nat pool net-internet 171.69.233.208 171.69.233.223 prefix-length 28
ip nat inside source list 1 pool net-internet
!
interface serial 0
Description wan access to internet
ip address
ip nat outside
!
interface ethernet 1
ip address b.b.b.b
ip nat inside
!
access-list 1 permit x.y.z.0 0.0.0.255
access-list 1 permit x1.y1.z1.a1 0.0.0.0
So you can see that the whole x.y.z.0/24 network have access to internet along with one ip address x1.y1.z1.a1