Study Guide for Computer Networks and OSI Model

Network

A collection of computers connected via software and hardware so that they can communicate with each other.

Resource Sharing

The ability to share hardware, software, and information across multiple computers on a network.

Network Administration

Tasks involved in managing a network, including installing new devices, monitoring performance, updating software, and configuring security.

Peer‑to‑Peer (P2P) Network

A network where each node can communicate directly with every other node; all nodes are peers.

Client/Server Network

A network with two distinct roles: Client: Requests resources or services (e.g., web page). Server: Provides resources or services and central administration (e.g., file storage, print services).

Local Area Network (LAN)

Nodes located within a small geographic area (e.g., computer lab).

Metropolitan Area Network (MAN)

Designed to provide network access across a city or large campus.

Wide Area Network (WAN)

Connects multiple LANs over long distances.

Medium

The physical channel (air, cable) through which communication travels.

Language

A common syntax or data format (e.g., HTTP, binary).

Protocol

A set of rules governing communication (e.g., the TCP three‑way handshake: SYN → SYN‑ACK → ACK).

Transmission Media

The physical means of connecting nodes (e.g., copper cable, fiber, wireless).

Network Interface Card (NIC)

Hardware that connects a computer to a network.

Router

Forwards packets between different networks (Layer 3).

Switch

Forwards frames within the same network segment (Layer 2).

Hub

Broadcasts incoming signals to all ports (Layer 1).

Repeater

Amplifies signals to extend reach (Layer 1).

Network Operating System (NOS)

Manages network resources and services.

Reference Model

A conceptual framework to standardize communication functions into seven layers.

OSI (Open Systems Interconnection) Model

Seven‐layer model to guide interoperable network product development.

TCP/IP vs. OSI

TCP/IP protocols underpin the Internet but don't map perfectly onto OSI's seven layers.

Encapsulation

Wrapping data with protocol information at each OSI layer, from Application down to Physical.

Physical Layer

Defines electrical, mechanical, and procedural interfaces to the transmission medium.

Modulation/Demodulation

Converting digital signals to analog (and back) for transmission (e.g., via a modem).

Flow Control

Regulating data transmission rate between devices.

Topology

The layout pattern of interconnections between nodes.

Star Topology

All nodes connect to a central hub/switch. Easy expansion; device isolation.

Ring Topology

Each node connects to two others; data circulates in one direction around the ring. Predictable performance.

Bus Topology

All nodes share a single backbone cable; only one node transmits at a time. Simple, cost‑effective.

HTTP

A protocol that operates at Layer 7 (Application Layer).

Layer 2 (Data Link Layer)

The layer that forwards frames based on MAC addresses and handles electrical signals at Layer 1.

Network Hub

A device that operates only at the Physical Layer, repeating incoming electrical signals to all other ports.

Star Topology

A network topology where all devices are connected to a central device.

Data Link Layer

Provides reliable transfer of data frames between two nodes connected by a physical layer, performing framing, physical addressing, error detection and handling, and controlling access to the shared medium.

Frame

The fundamental unit of data at Layer 2, encapsulating a network-layer packet with a header and trailer.

Frame Check Sequence (FCS) / CRC

A checksum value calculated over the frame's contents to ensure data integrity.

MAC Address (Media Access Control Address)

A 48-bit hardware address that uniquely identifies a network device on a local segment.

Organizationally Unique Identifier (OUI)

The first 24 bits of a MAC address assigned by the IEEE to NIC manufacturers.

Device Identifier

The remaining 24 bits of a MAC address, assigned uniquely by manufacturers within their OUI block.

Broadcast MAC Address

The special address FF:FF:FF:FF:FF:FF used to send a frame to all devices on the local network segment.

Switch

A Layer 2 network device that connects multiple devices on the same LAN and forwards frames only to the correct destination port.

Source Address Table (SAT) / MAC Address Table

An internal lookup table within a switch that records source MAC addresses and ingress ports.

Learning Process

When a frame arrives, the switch examines the source MAC and updates the SAT with the port on which it was received.

Forwarding Process

If the destination MAC is in the SAT, the switch forwards the frame only to the associated port; if unknown, it floods the frame to all ports except the source port.

Network Bridge

A device that connects two LAN segments at Layer 2 and maintains a MAC address table.

Wireless Access Point (WAP)

A bridge between wired and wireless networks that operates at Layer 2 by forwarding Ethernet frames.

Error Detection

Achieved via the FCS/CRC in each frame, where corrupted frames are silently dropped.

Flow Control

Mechanisms to prevent a fast sender from overwhelming a slow receiver, such as pause frames in Ethernet.

Half-duplex Environment

An environment where collisions occur due to devices not inspecting frames or learning addresses.

Cyclic Redundancy Check (CRC)

A method used to calculate the FCS value for error detection in frames.

Ingress Port

The port on which a frame is received by a switch.

Egress Port

The port to which a frame is forwarded by a switch.

Collisions

Events that occur in half-duplex environments when multiple devices attempt to send data simultaneously.

Ethernet (IEEE 802.3x)

A standard that includes flow control mechanisms like pause frames.

Ethernet Frame Formats

Defines several frame formats (e.g., Ethernet II, IEEE 802.3) that share common fields.

Preamble & Start Frame Delimiter (SFD)

Synchronization bits.

Destination MAC Address

6 bytes.

Source MAC Address

6 bytes.

Type/Length

Indicates EtherType or payload length.

Payload/Data

46-1500 bytes of encapsulated packet data.

FCS

4‑byte CRC.

Address Resolution Protocol (ARP)

Translates Layer 3 IPv4 addresses into Layer 2 MAC addresses on the same network.

ARP Request

A broadcast frame (FF:FF:FF:FF:FF:FF) asking, 'Who has IP x.x.x.x? Tell IP y.y.y.y.'

ARP Reply

A unicast frame from the host owning the queried IP, providing its MAC address.

ARP Cache

Both requester and responder store the IP‑to‑MAC mapping in a local table for a limited time.

ARP Probe

An ARP request sent by a host to check if a new or changed IP address is already in use.

Gratuitous ARP

An unsolicited ARP reply (or request) announcing a host's IP‑to‑MAC mapping.

ARP Spoofing

A malicious technique where an attacker sends fake ARP replies to poison other hosts' ARP caches.

MAC Address

Physical, burned into hardware, 48 bits, globally unique, used only on the local LAN segment.

IPv4 Address

Logical, assigned via DHCP or statically by software, 32 bits, routable across multiple networks.

Subnet Mask

A 32‑bit mask (e.g., 255.255.255.0) indicating which bits of the IPv4 address denote the network versus the host.

Framing

Encapsulate packets into frames with headers/trailers.

Physical Addressing

Use MAC addresses for source/destination identification.

Error Detection

Compute and verify CRC/FCS.

Flow Control

Manage data rate to prevent buffer overflow.

Medium Access Control

Coordinate multiple devices sharing the same medium (e.g., CSMA/CD in Ethernet).

Switching & Filtering

Forward frames based on MAC address learning and SAT lookups.

IPv4 Addresses

A 32‑bit number used to identify a host or network interface on an IP network.

Network and Host Portions

The 32 bits of an IPv4 address are divided into a network portion and a host portion.

Class A

0.0.0.0 - 127.255.255.255; First octet range: 0-127; Default netmask: 255.0.0.0 (/8); Network bits: first 8; Host bits: last 24; 128 possible networks, each supporting up to 16,777,214 hosts.

Class B

128.0.0.0 - 191.255.255.255; First octet range: 128-191; Default netmask: 255.255.0.0 (/16); Network bits: first 16; Host bits: last 16; 16,384 possible networks, each supporting up to 65,534 hosts.

Class C

192.0.0.0 - 223.255.255.255; First octet range: 192-223; Default netmask: 255.255.255.0 (/24); Network bits: first 24; Host bits: last 8; 2,097,152 possible networks, each supporting up to 254 hosts.

Class D

224.0.0.0 - 239.255.255.255; Reserved for multicast addresses (one‑to‑many distribution).

Class E

240.0.0.0 - 255.255.255.255; Reserved for experimental use.

Network Identifier

The first address in any network (all host bits zero).

Broadcast Address

The last address in any network (all host bits one).

Loopback

127.0.0.0/8; Any address from 127.0.0.0 to 127.255.255.255 loops back to the local host, regardless of physical interfaces.

Link-Local / APIPA

169.254.0.0/16; Automatically assigned by Windows and some OSes when DHCP fails, in the range 169.254.1.0-169.254.254.255.

Private Networks

10.0.0.0/8; 172.16.0.0/12; 192.168.0.0/16; These ranges are not routable on the public Internet and are used for internal LANs.

IPv6 Addresses

128‑bit addresses, written as eight groups of four hexadecimal digits, separated by colons (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334). Leading zeros in each 16‑bit group may be omitted.

Subnet Mask

A 32‑bit value that delineates which bits of an IPv4 address are the network portion (mask bits = 1) and which are the host portion (mask bits = 0).

Classless Inter-Domain Routing (CIDR)

CIDR supersedes rigid classful boundaries by allowing arbitrary mask lengths, appending /N to the address, where N is the count of leading 1 bits in the mask.

Default Gateway

The IP address of a router interface that a host uses to send packets destined for other networks.

Dynamic Host Configuration Protocol (DHCP)

Automates IP configuration for hosts, eliminating manual setup of IP address, subnet mask, default gateway, and DNS servers.

DHCP Discover

Client broadcasts DHCPDISCOVER.

DHCP Offer

One or more DHCP servers reply with DHCPOFFER, proposing an IP lease.

DHCP Request

Client selects an offer and broadcasts DHCPREQUEST.

DHCP Acknowledge

Server confirms with DHCPACK, finalizing the lease and providing configuration details.

DHCP Lease

Time‑limited; clients must renew before expiry.

MAC Address

The first three octets of a MAC address identify the manufacturer (OUI).