Networking for IT Officer

What the subject covers

Networking is about how data gets from one computer to another: the layered models that split the job up, the addresses that name each machine, the protocols at each layer and the devices that forward traffic. For an IT Officer it is also the base for security (firewalls, VPNs) and for running branch and ATM networks.

Key topics

Layered models

• OSI, 7 layers, top to bottom:
  • Application, Presentation (format, encryption), Session (dialogue control);
  • Transport (end-to-end delivery, ports), Network (logical addressing, routing);
  • Data Link (framing, MAC addresses, error detection), Physical (bits on the medium).
  • Mnemonic, bottom up: Please Do Not Throw Sausage Pizza Away.
• TCP/IP model: Application, Transport, Internet, Network Access (link). Know which OSI layers map to which.
• PDU names: bits (physical), frame (data link), packet (network), segment for TCP / datagram for UDP (transport), data/message (application).
• Transmission modes: simplex (one way), half duplex (both ways, one at a time), full duplex (both ways at once).

Data link layer

• Framing and error control.
  • Parity, checksum and CRC detect errors; Hamming code can correct single-bit errors.
  • Know whether a protocol is bit-oriented (HDLC) or character/byte-oriented.
  • Flow control: stop-and-wait, Go-Back-N and Selective Repeat (sliding window).
• Media access. Ethernet (IEEE 802.3) uses CSMA/CD on shared, half-duplex links (full-duplex switched Ethernet has no collisions); Wi-Fi (802.11) uses CSMA/CA, because a wireless station can't detect collisions while sending.
• MAC addresses are 48 bits, written in hex.
• Ethernet cabling. A 100BASE-TX segment over twisted pair can be up to 100 m long.
• VLANs. A VLAN splits one switch into separate broadcast domains. A trunk link carries several VLANs, tagged with IEEE 802.1Q.

Network layer: IP addressing

• IPv4. 32 bits, dotted decimal. Classful ranges (first octet):

| Class | First octet | Default mask | |---|---|---| | A | 1–126 | 255.0.0.0 (/8) | | B | 128–191 | 255.255.0.0 (/16) | | C | 192–223 | 255.255.255.0 (/24) | | D | 224–239 | multicast | | E | 240–255 | reserved |

127.x.x.x is loopback.

• Private ranges: 10.0.0.0/8, 172.16.0.0/12 (172.16–172.31), 192.168.0.0/16. Also know APIPA, 169.254.x.x.
• Subnetting and CIDR.
  • Borrowing n host bits gives 2ⁿ subnets. Older books use 2ⁿ − 2 subnets (no subnet zero); if both 2ⁿ and 2ⁿ − 2 appear in the options, the question follows the old classful rule.
  • With h host bits left, each subnet has 2ʰ − 2 usable hosts: the network and broadcast addresses are reserved.
  • Example: a Class B (/16) that needs at least 300 subnets borrows 9 bits, giving /25 = 255.255.255.128, 512 subnets and 126 hosts each.
• IPv6.
  • 128 bits, written in hex in 8 groups; :: shortens one run of zero groups.
  • There's no broadcast (multicast and anycast instead) and the base header is a fixed 40 bytes.
• Helper protocols.
  • ARP finds the MAC address for an IP on the local network; RARP did the reverse.
  • ICMP carries error and diagnostic messages, used by ping and traceroute.
  • NAT translates private addresses to a public one, so many devices share one public IP and their internal addresses stay hidden.
  • DHCP hands out IP settings automatically.
• Routing.
  • Static vs dynamic.
  • Distance vector: RIP, hop count, maximum 15 hops, Bellman-Ford.
  • Link state: OSPF, Dijkstra's shortest path.
  • Path vector: BGP, between autonomous systems on the internet.

Transport layer

• TCP.
  • Connection-oriented and reliable: a 3-way handshake (SYN, SYN-ACK, ACK), sequence numbers, acknowledgements and retransmission.
  • Flow control with a sliding window; congestion control with slow start and congestion avoidance.
• UDP: connectionless, no guarantees, low overhead. Used for DNS queries, DHCP, streaming and VoIP.
• Ports:

| Port | Protocol | |---|---| | 20/21 | FTP | | 22 | SSH | | 23 | Telnet | | 25 | SMTP | | 53 | DNS | | 67/68 | DHCP | | 80 | HTTP | | 110 | POP3 | | 143 | IMAP | | 161 | SNMP | | 443 | HTTPS |

Application layer

• HTTP/HTTPS.
  • HTTP methods: GET, POST, PUT, DELETE.
  • Status code classes: 1xx informational, 2xx success, 3xx redirect, 4xx client error, 5xx server error.
  • HTTP is stateless (cookies add state); HTTPS is HTTP over TLS.
• DNS: turns names into IP addresses. Know record types A, AAAA, MX, CNAME, NS, and recursive vs iterative queries.
• Email. SMTP sends mail and runs over TCP. POP3 usually downloads mail and removes it from the server. IMAP keeps mail on the server and syncs folders and read status across devices.
• FTP: separate control and data connections. Transfer modes: stream, block, compressed.

Delays and performance

• Transmission delay = packet size ÷ link rate (L/R). Propagation delay = distance ÷ signal speed.
• Store-and-forward. Each router must receive the whole packet before sending it on. Through N routers there are N + 1 links, so the transmission delay alone is (N + 1) × L/R.
• Also know bandwidth vs throughput, and the bandwidth-delay product.

Devices and topologies

• Devices by layer.
  • Hub and repeater: layer 1, one collision domain.
  • Switch and bridge: layer 2. Each switch port is its own collision domain; switches can mirror a port for monitoring.
  • Router: layer 3, separates broadcast domains.
  • Gateway: connects different protocols.
• Topologies: bus, star, ring, mesh (n(n−1)/2 links for full mesh), tree, hybrid. Also know LAN, MAN and WAN.

How it is asked

These are the facts about the format from the official IBPS documents:

• Professional Knowledge in the Mains has 50 questions for 100 marks in 40 minutes (2 marks each, −0.50 for a wrong answer).
• Each question has five options. In IBPS's sample, the fifth is "Other than those given as options".

IBPS doesn't publish past papers. Based on memory-based papers shared by candidates (unofficial) for two previous IT Officer Mains exams (January 2023 and December 2024; possibly incomplete), networking questions were of three kinds:

• Address arithmetic: the class and first-octet range of an address, how many networks a class has, picking a private address, and the subnet mask needed for a given number of subnets.
• Protocol facts in one line: which transport protocol a service uses, which protocol is bit-oriented, what ARP and NAT do, a feature of IMAP, FTP transfer modes, what a traceroute option does, PDU names per layer, and matching OSI layers to their jobs.
• Devices, standards and calculations: the maximum length of a 100BASE-TX segment, trunk VLANs, which device supports port mirroring, full duplex, and a store-and-forward delay formula.

Firewall types and network attacks also came up; they are covered under Information and cyber security. We don't give a number of networking questions, because IBPS publishes no weightage.

Common mistakes

• Forgetting the "− 2" for hosts. It applies to hosts per subnet, not to the number of subnets (unless the question follows the old 2ⁿ − 2 subnet rule).
• Mixing up mask and CIDR. /26 = 255.255.255.192 (64 addresses, 62 usable). Practise /24 to /30 until they're automatic.
• Classful first-octet ranges. 127 belongs to no class (loopback), and Class C starts at 192, not 191.
• "DNS uses only UDP." DNS queries normally use UDP 53, but large responses and zone transfers use TCP.
• POP3 vs IMAP. IMAP keeps mail on the server and syncs it.
• Layer of a device. A switch is layer 2 (MAC). A "layer 3 switch" also routes, but the default answer for "switch" is layer 2.
• Store-and-forward links. N routers means N + 1 links, not N.
• CSMA/CD vs CSMA/CA: wired vs wireless.

Two-week study plan

A suggestion. Adjust it to your own pace.