BREB Assistant Programmer Exam: 18.02.2023প্রশ্ন ও সমাধান

Direct answer: RFC 1918 reserves one private block from each of classes A, B, and C for use inside organizations, never routed on the public internet.

Class	Private Range	Default Mask	Host bits	Max hosts per network	Networks in the block
A	10.0.0.0 – 10.255.255.255	255.0.0.0 (/8)	24	2^24 - 2 = 16,777,214	1 single /8 network
B	172.16.0.0 – 172.31.255.255	255.255.0.0 (/16)	16	2^16 - 2 = 65,534	16 contiguous /16 networks
C	192.168.0.0 – 192.168.255.255	255.255.255.0 (/24)	8	2^8 - 2 = 254	256 contiguous /24 networks

Why "-2" in the host count: for any network, the all-zero host bits address is the network address and the all-one host bits address is the broadcast address — neither is assignable to a device, so usable hosts = 2^(host bits) - 2.

Why these specific ranges exist: IPv4's roughly 4 billion public addresses ran out faster than expected, so RFC 1918 carved out address space that any organization can reuse internally without conflicting with anyone else's internal network, since private addresses are never forwarded across the public internet (a router or NAT gateway translates them to a public address before traffic leaves the organization).

Practical example: a home router typically hands out addresses from 192.168.1.0/24 (Class C block) to laptops and phones, while a large enterprise with tens of thousands of devices would more likely use the Class A block 10.0.0.0/8 for its sheer address-space size.

Exam takeaway: memorize the three ranges by their class boundary (10/8, 172.16-172.31/16, 192.168/24) and the standard "-2" adjustment for network + broadcast addresses when computing usable hosts.

Direct answers, each with its role:

graph LR
  N1[Node: PC] ---|Link: Ethernet| SW[Node: Switch]
  SW ---|Link: Fiber| GW[Gateway/Router]
  GW ---|Link: WAN| CLOUD[(Internet)]

• Network: a collection of interconnected devices (computers, servers, printers) that can exchange data and share resources; classified by geographic scope into LAN (single building/site), MAN (city-wide), and WAN (spans countries/continents, e.g. the internet).
• Protocol: an agreed set of rules governing how data is formatted, transmitted, and error-checked between devices — e.g. TCP/IP defines addressing and reliable delivery, HTTP defines how web requests/responses are structured. Without a shared protocol, two devices cannot meaningfully exchange data even if physically connected.
• Link: the direct physical or logical connection between two adjacent network devices — physical links include Ethernet cable, fiber, or Wi-Fi; logical links include a VPN tunnel or VLAN that behaves like a dedicated connection over shared physical infrastructure.
• Gateway: a device (often a router) that connects two networks using different protocols or architectures, translating between them — e.g. the device that connects a home LAN to the ISP's WAN, converting private addressing to a public one.
• Node: any addressable device on a network capable of sending or receiving data — computers, routers, switches, and printers are all nodes, each identified by a unique address (MAC and/or IP).

How they relate: nodes are connected by links; links carry data according to a shared protocol; many nodes and links together form a network; a gateway is the special node that lets one network talk to a structurally different network.

Exam takeaway: node = a device; link = the connection between two nodes; protocol = the rules data must follow over that link; network = the whole interconnected system; gateway = the translator between two different networks.