Telephone Shilpa Sangstha Ltd. (TSS) Assistant Programmer Exam: 2022প্রশ্ন ও সমাধান

NAT (Network Address Translation): a router/firewall function that rewrites the source (and/or destination) IP address — usually together with the port — of packets as they cross between a private network and a public network (the Internet). It lets many devices using private IP addresses share one, or a few, public IP address(es), which both conserves scarce IPv4 address space and hides the internal network's topology from the outside world.

Common types:

• Static NAT — one private IP permanently mapped to one public IP.
• Dynamic NAT — private IPs drawn from a pool of public IPs on demand.
• PAT / NAPT (overload) — many private IPs share one public IP, distinguished by port number; this is what home/office routers use.

Private IP address ranges (RFC 1918):

Class	Range	CIDR
A	10.0.0.0 – 10.255.255.255	10.0.0.0/8
B	172.16.0.0 – 172.31.255.255	172.16.0.0/12
C	192.168.0.0 – 192.168.255.255	192.168.0.0/16

How it works:

graph LR
    H1[Host 10.0.0.2] --> R[NAT Router]
    H2[Host 10.0.0.3] --> R
    R -->|Public IP 203.0.113.5, per-connection ports| I((Internet))

The router maintains a translation table (private IP:port ↔ public IP:port) so return traffic is routed back to the correct internal host.

Exam takeaway: NAT = private-to-public IP/port translation at the network edge; the three reserved private blocks are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 — none of these are routable on the public Internet.