62 Networking and Online Games: Understanding and Engineering
62 Networking and Online Games: Understanding and Engineering Multiplayer Internet Games The router remembers the address mapping it used, so it can reverse the process for inbound packets coming from the ISP. For packets coming back in from the Internet, the reverse steps are as follows: A packet arrives at the NAT-enabled router, with a destination address in the 128.80.6/24 range. The NAT-enabled router looks up the mapping between 128.80.6/24 addresses and internal 192.168.0.*, and replaces the packet s destination address with the private IP address of the intended destination host. Internal routing (within the private network) forwards the modified IP packet to the correct destination. A NAT-enabled router is generally free to use whatever address mapping schemes it chooses. For example, in Figure 4.16 the NAT-enabled router might choose to map internal address 192.168.0.10 to public address 128.80.6.20, or indeed any address in the 128.80.6/24 range. Mappings may be statically assigned, or dynamically generated on-demand. The only requirement is that mappings are unique multiple private host addresses should never map to the same public IP address, and vice versa. 4.3.2.2 Network Address Port Translation A common scenario for home networks is where the ISP (whether regular modem dialup or a broadband service) charges additional monthly fees for a second or third public IP address. The solution is an extended version of NAT called Network Address Port Translation (NAPT) [RFC3022]. An NAPT-enabled router transparently makes multiple hosts on the private network appear to be a single host from the perspective of the public Internet. NAPT extends NAT by additionally manipulating the port numbers of TCP and UDP traffic going in and out of the private network. Consider the scenario of Figure 4.17, where two hosts on a private LAN (192.168.0.12 and 192.168.0.13) are sharing a single public IP address (128.80.6.200). Home subnet 192.168.0/24 192.168.0.13 192.168.0.12 NAPT-enabled router ISP All outbound packets have source address 128.80.6.200 192.168.0.1128.80.6.200 Addresses and ports are re-mapped on the way to ISP Source 192.168.0.12:W becomes Source 128.80.6.200:Y Source 192.168.0.12:X becomes Source 128.80.6.200:Z Figure 4.17 NAPT maps both addresses and TCP/UDP ports to share public IP addresses across multiple private hosts
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