rfc 1918 (04/06/13 05:18:05)
it specifies the private address subnets, which are not publicly routed.
home routers have a std default cfg in which they choose a small fraction of this address space and consider it local (private). router has an ip address on this private nonroutable space (say 192.168.1.1/24)
every ip address (ipv4 addresses are 32 bits, 4 octects) can be understood as being composed by a network part and a host part. network address is the left part. each part is made by a number of bits, summing to 32. a common solution for home networks is to use /24 subnetting, which means the first 24 bits are for the network and the last 8 bits are for the host. first and last address of each ip network are reserved to be network address and broadcast address (in /24 subnets these are addresses ending in .0 and .255). adjacent addresses are normally used for routers (in /24 subnets these are addresses ending in .1 and .254)
so the router has an ip address on the local space (say 192.168.1.1/24) and an address on the external internet space (say 18.104.22.168). the external address (PRA=publicly routable address) is said to be routable on the internet because the routers in the wild know where to send packets addressed to it.
traffic flow going from your private network TO the internet is translated in the router so that the original source address is changed into PRA when packet is copied on the external internet (so that its replies know how to come back). Router keeps a dynamic table for this translation (NAT=network address translation) and translation requires also changing TCP/UDP service ports to keep a meaningful bidirectionally maneageable translation table.
destination addresses of inbound traffic coming FROM the internet as a reply to previous matching outgoing requests is processed accordingly (translated back) and routed to the requester.
static NAT is for coupling specific port services that you want to expose on your PRA to specific hosts in your private space (for example your webserver).
NATs can be nested, and this allows your provider assigning non public addresses to your router external interface.
NAT was invented as a countermeasure for public ipv4 address space exaustion, and is now ubiquitous. it is probably the greatest obstacle against the diffusion of the next generation ip protocols ipv6).
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