If an ISP deploys a CGN .
If an ISP uses RFC 1918 address space to number customer gateways , the risk of address collision , and therefore routing failures , arises when the customer network already uses an RFC 1918 address space .
If an ISP deploys a CGN , and uses RFC 1918 address space to number customer gateways , the risk of address collision , and therefore routing failures , arises when the customer network already uses an RFC 1918 address space .