Computer Network (BCA 301): A Study Guide

Computer Network (BCA 301) is a three-credit course in the fifth semester, with three lecture hours and three practical hours a week. It follows Operating Systems and sits alongside Artificial Intelligence, Advance Java Programming and Project-II. The fifth semester guide shows how those subjects compete for your time.

Almost everything in the course can be tested on your own machine. You can watch a name resolve, follow a request to a web server, and inspect the packets that result, which makes it one of the most satisfying BCA subjects to study properly.

Layers, and why they exist

Layering divides a large problem so that each part can change without breaking the others. The OSI reference model describes seven layers, from the physical medium up through data link, network, transport, session, presentation and application. The TCP/IP model that the internet runs on groups the same concerns into fewer layers, and you meet the same ideas under both names.

For each layer, learn three things: the unit of data it handles, the addressing it uses and the problem it solves. A frame travels within a local network using a hardware address; a packet travels between networks using an IP address; a segment carries port numbers; a message is what the application cares about. Most confusion in this subject comes from mixing up those units.

IP addressing and subnetting

An IPv4 address is 32 bits written as four decimal numbers. A prefix such as /24 says how many leading bits identify the network, and the remainder identify the host. The subnet mask is simply that prefix written out in full: /24 corresponds to 255.255.255.0. Two addresses in the same prefix are local to each other; anything else has to go through a router.

A worked example

Take 192.168.1.0/26. The prefix length 26 leaves 6 bits for hosts, so the block holds 64 addresses (2 to the power 6). The first address is the network address and the last is the broadcast address, which leaves 62 usable host addresses. The next block starts at 192.168.1.64. Being able to produce a table of blocks like that in under a minute is worth more in an exam than any amount of description.

Two more ideas belong here. First, private address ranges are reused inside organisations and reach the internet through address translation, which is why many different networks can share the same internal numbering. Second, IPv6 uses 128-bit addresses and a different notation, and it exists because the IPv4 space is exhausted.

Routing

A router forwards a packet towards its destination using a routing table. Static routes are configured by hand and suit small, stable networks. Dynamic routing protocols let routers exchange information and adapt: distance vector protocols such as RIP share a cost to each known network with their neighbours, while link state protocols such as OSPF let every router build a map of the topology and compute shortest paths from it.

Learn how a routing table lookup works, what the longest matching prefix means, and the difference between interior protocols used inside one organisation and exterior protocols used between them. A few routers in a lab or a simulator show this clearly; you do not need a large network to understand it.

TCP and UDP

TCP provides a reliable, ordered byte stream. It establishes a connection, numbers every byte, acknowledges what it receives, retransmits what is lost, controls its sending rate and closes the connection in an orderly way, all of which costs setup time and delay. UDP sends independent datagrams with no connection and no delivery guarantee, which suits short queries, real-time audio and applications that handle their own retries.

DNS, HTTP and the web

The domain name system translates names into addresses. A resolver checks its cache, then asks a root server, then the server for the top-level domain, then the authoritative server for the zone. Answers are cached for a period set by the record, which is why a change of address takes time to spread.

HTTP is a request and response protocol. A request names a method, a path and headers; a response carries a status code and a body. Codes in the 200 range mean success, 300 means redirection, 400 means a problem with the request and 500 means a problem at the server. HTTP is stateless, so cookies and tokens are used to keep track of a session. Open your browser developer tools, load a page and read the requests; that single exercise teaches more than a chapter describing the same thing.

Basic security

At this level, security is about three properties: confidentiality, integrity and availability. Encryption protects confidentiality. Symmetric ciphers such as AES use one shared key and are fast; asymmetric schemes use a public and a private key and solve the problem of agreeing on a key in the first place. Hash functions give a fixed-length fingerprint, which is how change is detected.

Transport layer security combines those pieces: a handshake authenticates the server with a certificate, both sides agree a shared key, and the rest of the session is encrypted. Firewalls filter traffic by rule, and a simple access control list on a router is a good first exercise.

How to prepare

Divide your revision by layer and keep a table of addressing calculations: prefixes, block sizes, usable host counts, network and broadcast addresses. Practise the numerical parts until they are quick, then spend the rest of your time on what happens when something fails.

Your practical file should show what you actually configured or observed, with the commands you used and the results you obtained. Practicals that record real output are far more convincing than a rewritten theory answer, and the lab report format guide explains how the cover page and index are normally laid out.

Tool names, commands and the exact practical list vary between campuses, so check the list issued by your department rather than working from a friend in another college.