IDRA Assistant Network Administrator Exam: 2022প্রশ্ন ও সমাধান

Direct answer: Network Address Translation (NAT) is a router function that rewrites the source/destination IP address (and often port) of packets crossing between a private network and the public Internet, letting many devices with private IP addresses share one or a few public IP addresses.

Why it's needed:

• IPv4's address space is far smaller than the number of devices online — NAT lets an entire LAN (using private ranges like 192.168.x.x) reach the Internet through one public IP, conserving address space.
• It also hides internal addressing from the outside world, adding a basic layer of security.

How it works:

graph LR
    PC1["PC 192.168.1.10"] --> R["NAT Router - Public IP 203.0.113.5"]
    PC2["PC 192.168.1.11"] --> R
    R --> Internet((Internet))

The router keeps a translation table mapping (private IP, private port) to (public IP, public port) for each connection, so return traffic reaches the correct internal device.

Types:

• Static NAT — one private IP permanently mapped to one public IP.
• Dynamic NAT — private IPs mapped to a pool of public IPs on demand.
• PAT / NAPT (overloading) — many private IPs share one public IP, distinguished by port number; used by most home/office routers.

Example: A home router with one ISP-assigned public IP lets a laptop, phone, and smart TV all browse the Internet simultaneously via PAT, each session tracked by a distinct port.

Exam takeaway: NAT conserves public IPv4 addresses and hides private addressing by translating private-to-public IP (and port) mappings — PAT/NAPT is the most common form.

Direct answer: A public IP is globally unique and routable directly on the Internet; a private IP is unique only within its own LAN and cannot be routed on the public Internet — it needs NAT to reach the outside.

Aspect	Public IP	Private IP
Uniqueness	Globally unique	Unique only within its own LAN
Assigned by	ISP / regional registry	Router/DHCP server on the local network
Routable on Internet	Yes, directly	No — needs NAT
Reserved ranges	Everything outside private ranges	10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 (RFC 1918)
Cost	Often limited/charged by ISP	Free, reusable across every private network
Example use	Web servers, public DNS, email servers	Home/office PCs, printers, IoT devices

Example: A home router might have public IP 103.10.20.5 from the ISP facing the Internet, while your laptop behind it gets a private IP like 192.168.0.12 — NAT on the router translates between the two so your laptop can browse the web.

Exam takeaway: Private IP ranges (10/8, 172.16/12, 192.168/16) are reserved and reused in every private network; a public IP must be globally unique, and NAT is what translates private addresses into public ones for Internet access.