Cheat Sheet & Glossary

Common port numbers

Port	Protocol	Description
20, 21	TCP	FTP data and control
22	TCP	SSH (Secure Shell)
23	TCP	Telnet (insecure, avoid)
25	TCP	SMTP (mail server to server)
53	UDP/TCP	DNS
67, 68	UDP	DHCP (server, client)
80	TCP	HTTP
110	TCP	POP3 (mail retrieval)
143	TCP	IMAP (mail retrieval)
161, 162	UDP	SNMP (monitoring)
443	TCP	HTTPS
465, 587	TCP	SMTPS / SMTP submission
993, 995	TCP	IMAPS / POP3S
3306	TCP	MySQL
5432	TCP	PostgreSQL
6379	TCP	Redis
8080, 8443	TCP	HTTP/HTTPS alt (dev/proxies)

IPv4 subnet quick reference

Prefix	Mask	Addresses	Usable hosts
/8	255.0.0.0	16,777,216	16,777,214
/16	255.255.0.0	65,536	65,534
/24	255.255.255.0	256	254
/25	255.255.255.128	128	126
/26	255.255.255.192	64	62
/27	255.255.255.224	32	30
/28	255.255.255.240	16	14
/29	255.255.255.248	8	6
/30	255.255.255.252	4	2
/31	255.255.255.254	2	2 (point-to-point)
/32	255.255.255.255	1	1 (host route)

Glossary

ACK: Acknowledgement flag in TCP; confirms receipt of data.
ARP: Address Resolution Protocol — maps IP addresses to MAC addresses on a LAN.
AS: Autonomous System — a network under one administrative control, identified by an ASN.
BGP: Border Gateway Protocol — inter-AS routing protocol of the internet.
CIDR: Classless Inter-Domain Routing — IP addressing using prefix lengths (e.g. /24).
DHCP: Dynamic Host Configuration Protocol — automatically assigns IP config to hosts.
DNS: Domain Name System — hierarchical distributed database mapping names to IPs.
ECDHE: Elliptic Curve Diffie-Hellman Ephemeral — key exchange with forward secrecy.
FIN: TCP flag indicating the sender has finished sending data (begins teardown).
FCS: Frame Check Sequence — CRC-32 trailer in Ethernet frames for error detection.
ICMP: Internet Control Message Protocol — diagnostic/error protocol used by ping and traceroute.
ISN: Initial Sequence Number — random starting point for a TCP connection's byte numbering.
MAC: Media Access Control address — 48-bit hardware identifier for a network interface.
MTU: Maximum Transmission Unit — largest frame/packet a link can carry (Ethernet: 1500 bytes).
NAT: Network Address Translation — maps private IP addresses to one or more public IPs.
OSPF: Open Shortest Path First — link-state intra-domain routing protocol.
PDU: Protocol Data Unit — the name for a unit of data at each layer (segment, packet, frame).
QUIC: Quick UDP Internet Connections — modern transport protocol underpinning HTTP/3.
RFC: Request for Comments — IETF standards document that defines internet protocols.
RTT: Round-Trip Time — time for a packet to travel to a destination and back.
SACK: Selective ACKnowledgement — TCP option letting the receiver report non-contiguous data.
SYN: Synchronise flag in TCP — used to initiate a connection.
TLS: Transport Layer Security — cryptographic protocol providing encryption over TCP.
TTL: Time To Live — IP field decremented at each hop; prevents routing loops.
UDP: User Datagram Protocol — connectionless, unreliable, low-overhead transport protocol.
VLAN: Virtual LAN — logical segmentation of a switch into multiple broadcast domains.
VPC: Virtual Private Cloud — isolated virtual network in a cloud provider's infrastructure.
VPN: Virtual Private Network — encrypted tunnel connecting two network endpoints.

Core Exam Formulas Quick Reference

The key mathematical formulas, bounds, and protocol timer rules frequently tested on computer networking exams:

Topic	Formula / Rule	Variable Definitions & Key Units	Exam Context & Notes
1. Transmission Delay	`d_trans = L / R`	`L` = packet size (bits) `R` = link bandwidth (bps)	Time required to push all packet bits onto the physical medium. Independent of distance.
2. Propagation Delay	`d_prop = d / s`	`d` = distance (metres) `s` ≈ `2 × 10⁸ m/s` (speed of light in copper/fibre)	Time for a bit to physically travel from source to destination. Independent of packet size and link rate.
3. Total Nodal Delay	`d_nodal = d_proc + d_queue + d_trans + d_prop`	Additive delay components at a single router hop	`d_proc` is typically µs; `d_queue` is dynamic and depends on traffic intensity.
4. Traffic Intensity	`I = (L × a) / R`	`L` = bits/packet `a` = arrival rate (packets/s) `R` = link rate (bps)	If `I → 1`, queuing delay increases exponentially. If `I > 1`, buffer overflows and packets drop.
5. Multi-Packet Pipelining (Store-and-Forward)	`d_total = (N + P − 1) × (L / R)`	`P` = number of packets `N` = number of identical links in series `L / R` = transmission delay per link	Accounts for overlapping packet transmissions across consecutive hops (ignoring propagation/queuing delay).
6. Usable IPv4 Subnet Hosts	`Usable Hosts = 2^(32 − n) − 2`	`n` = prefix length (e.g. `/24` → `32 − 24 = 8` host bits)	Subtract 2 for the all-zeros network address and all-ones directed broadcast address. (Exception: `/31` per RFC 3021).
7. IPv4 Fragment Offset	`Offset = Byte Offset ÷ 8`	Fragment offset field in IPv4 header is measured in 8-byte blocks	Every fragment's payload (except the last) must be an integer multiple of 8 bytes. More Fragments (`MF = 1`) for all but last.
8. TCP Effective Sending Window	`EffectiveWindow = min(cwnd, rwnd)` `rwnd = RcvBuffer − (LastByteRcvd − LastByteRead)`	`cwnd` = congestion window (sender estimated) `rwnd` = flow control window (receiver advertised)	At any moment, in-flight unacknowledged bytes cannot exceed `min(cwnd, rwnd)`.
9. TCP Window-Limited Throughput	`Throughput ≤ Window / RTT`	`Window` in bits `RTT` in seconds	Upper bound on throughput for a single TCP connection regardless of link bandwidth.
10. TCP AIMD Average Throughput	`Throughput_avg ≈ 0.75 × (W_max × MSS) / RTT`	`W_max` = peak congestion window (in packets) `MSS` = maximum segment size	Congestion window oscillates between `W/2` and `W` under periodic packet loss; average window is `0.75 W`.
11. HTTP Page Load Latency (1 HTML + M Images)	• Non-persistent sequential: `2(M + 1) RTT` • Non-persistent parallel (K conns): `2 + 2 × ceil(M / K) RTT` • Persistent (no pipelining): `(M + 2) RTT` • Persistent pipelined: `3 RTT`	`M` = number of embedded images/objects `K` = number of concurrent parallel TCP sockets	Assumes negligible transmission delay and identical server origin for all resources.
12. DHCP Lease Timers (RFC 2131)	• Renewal (T1): `50% × LeaseTime` • Rebinding (T2): `87.5% × LeaseTime`	Client state machine timer defaults	At T1, client unicasts DHCPREQUEST to leasing server. At T2, client broadcasts DHCPREQUEST to any server.

Cheat Sheet & Glossary

Common port numbers

IPv4 subnet quick reference

Glossary

Core Exam Formulas Quick Reference

Further reading