Concept of Computer Network and Network Security System

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This section covers networking models, protocols and standards, the OSI and TCP/IP reference models, common networking devices, and transmission media used at the physical layer.

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Networking Models & Standards A protocol is a set of rules governing how data is formatted, transmitted, and interpreted between communicating devices; standards (de facto or de jure) ensure interoperability between different vendors' equipment.

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A networking (reference) model organizes network functions into layers, each with a defined responsibility, simplifying design and enabling independent development of each layer.

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OSI Model vs TCP/IP Model OSI Model (7 layers) TCP/IP Model (4 layers) Application, Presentation, Session Application Transport Transport Network Internet Data Link, Physical Network Access (Link) The OSI model is a conceptual/theoretical 7-layer reference model; the TCP/IP model is the practical, simpler 4-layer model that underlies the actual Internet.

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Networking Devices Device OSI Layer & Function Hub Physical layer; broadcasts incoming signal to all ports, no intelligence, shared collision domain Bridge Data Link layer; connects two LAN segments, filters traffic by MAC address, reduces collision domains Switch Data Link layer; multi-port bridge, forwards frames only to the intended port based on MAC address table, greatly improving efficiency Router Network layer; forwards packets between different networks based on IP address, using routing tables/protocols Transmission Media Medium Characteristic Twisted pair cable Copper pairs twisted to reduce interference; low cost, moderate bandwidth (e.g.

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Ethernet cabling) Coaxial cable Central conductor + shield; better noise immunity and bandwidth than twisted pair, used historically for cable TV/early Ethernet Optical fiber Light signals through glass/plastic fiber; very high bandwidth, immune to electromagnetic interference, low attenuation over long distances Wireless (radio/microwave) No physical medium; signal propagates through air, susceptible to interference/attenuation, enables mobility

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This section covers data link layer services, error detection/correction, flow control, multiple access protocols, LAN addressing, key IEEE 802 standards, wireless LANs, and wide-area/point-to-point protocols.

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Services & Error Control Function Description Framing Organizes raw bits into discrete frames with defined boundaries for transmission Error detection Identifies transmission errors using techniques like parity check, checksum, or Cyclic Redundancy Check (CRC) Error correction Can additionally fix detected errors (e.g.

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Hamming code), rather than only flagging them Flow control Prevents a fast sender from overwhelming a slow receiver (e.g. stop-and-wait, sliding window) Multiple Access Protocols & LAN Addressing Multiple access protocols coordinate how multiple stations share a common transmission medium:

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CSMA/CD (Carrier Sense Multiple Access with Collision Detection — classic Ethernet, station listens before/while transmitting and stops on collision), token passing (a token circulates, only its holder may transmit).

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LAN addressing: each network interface has a unique MAC (physical) address;

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ARP (Address Resolution Protocol) maps an IP address to its corresponding MAC address on the local network.

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IEEE 802 Standards Standard Technology IEEE 802.3 Ethernet — CSMA/CD-based LAN standard, the dominant wired LAN technology IEEE 802.4 Token Bus — bus topology with token-passing medium access IEEE 802.5 Token Ring — ring topology with token-passing medium access Wireless LANs & WAN Protocols Wireless LANs (IEEE 802.11 family) use CSMA/CA (Collision Avoidance, since collision detection is impractical over radio) for medium access.

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PPP (Point-to-Point Protocol): a data link protocol for establishing a direct connection between two nodes, commonly used for dial-up and WAN links; provides framing, authentication, and error detection.

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Wide area protocols (e.g.

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Frame Relay, ATM, PPP) handle data link functions over long-distance links connecting geographically dispersed networks.

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This section covers IP addressing and subnetting, major routing protocols and algorithms, supporting protocols (ARP, RARP, ICMP), and the transition to IPv6.

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Addressing & Subnetting An Internet (IP) address uniquely identifies a device on a network; classful addressing (historical) divided the address space into classes A, B, C (for unicast, decreasing host-count/increasing network-count) plus D (multicast) and E (reserved).

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Subnetting divides a single network into smaller sub-networks by borrowing bits from the host portion of an address for additional network identification, improving address utilization and network organization.

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Routing Algorithms Algorithm Approach Shortest path algorithm (Dijkstra) Computes the least-cost path between nodes using a graph model of the network Flooding Every incoming packet is sent out on every outgoing link except the one it arrived on; robust but generates heavy redundant traffic Distance vector routing Each router shares its routing table (distance estimates) with neighbors periodically; simple but can suffer slow convergence (count-to-infinity problem) Link state routing Each router floods information about its directly connected links to all routers, enabling each to independently compute shortest paths (e.g. via Dijkstra); faster convergence than distance vector Routing Protocols Protocol Type / Use RIP (Routing Information Protocol) Distance-vector interior gateway protocol; simple, uses hop count as metric, limited to small networks OSPF (Open Shortest Path First) Link-state interior gateway protocol; faster convergence, scales to larger networks BGP (Border Gateway Protocol) Exterior gateway (path-vector) protocol; used for routing between autonomous systems on the Internet Supporting Protocols & IPv6 ARP resolves IP→MAC;

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RARP (Reverse ARP) resolves MAC→IP (mostly superseded by DHCP);

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ICMP (Internet Control Message Protocol) reports errors and diagnostic information (e.g. ping, traceroute).

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128-bit addressing (vs IPv4's 32-bit), simplified/extensible header format using extension headers, built-in support for multicasting, and mechanisms (dual-stack, tunneling, translation) for transitioning from IPv4.

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This section covers the transport layer's service model, port/socket addressing, connection establishment and release, flow control, multiplexing, and congestion control algorithms.

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Transport Service & Protocols Protocol Service Type TCP (Transmission Control Protocol) Connection-oriented, reliable, ordered byte-stream delivery with flow/congestion control — used where reliability matters (web, email, file transfer) UDP (User Datagram Protocol) Connectionless, unreliable, low-overhead datagram delivery — used where speed matters more than reliability (streaming, DNS queries, VoIP) Ports & Sockets A port number identifies a specific application/process on a host (e.g. port 80 for HTTP, port 443 for HTTPS); well-known ports (0–1023) are reserved for standard services.

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A socket is the combination of an IP address and a port number, uniquely identifying one endpoint of a network connection.

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Connection Establishment & Release TCP uses a three-way handshake (SYN, SYN-ACK, ACK) to establish a connection, synchronizing sequence numbers between both endpoints.

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Connection release commonly uses a four-way handshake (FIN/ACK exchanges from each side), since TCP connections are full-duplex and each direction is closed independently.

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Flow Control, Multiplexing & Congestion Control Flow control (e.g.

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TCP's sliding window) prevents a fast sender from overwhelming the receiver's buffer.

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Multiplexing/de-multiplexing: the transport layer combines data from multiple application processes onto the network layer (multiplexing) and delivers incoming data to the correct process based on port number (de-multiplexing).

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Congestion control algorithms (e.g.

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TCP's slow start, congestion avoidance, fast retransmit/fast recovery) regulate how much data is injected into the network to avoid overwhelming intermediate routers/links, distinct from flow control (which protects the receiver, not the network).

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This section covers key application-layer protocols and services — web, file transfer, email, DNS, peer-to-peer applications, socket programming, and network traffic analysis tools.

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Web, File Transfer & Email Protocol/Tool Purpose HTTP / HTTPS HyperText Transfer Protocol (and its secure, TLS/SSL-encrypted variant HTTPS) for retrieving/transferring web content FTP File Transfer Protocol; transfers files between client and server, typically using separate control and data connections PuTTY A terminal client commonly used for SSH/Telnet remote login sessions WinSCP A graphical SFTP/SCP/FTP client for secure file transfer, commonly used on Windows SMTP / POP3 / IMAP Electronic mail:

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SMTP sends mail between servers;

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POP3/IMAP retrieve mail from a mail server to a client (IMAP keeps mail synced on the server, POP3 typically downloads and removes it) DNS, P2P & Socket Programming DNS (Domain Name System) translates human-readable domain names into IP addresses, using a distributed, hierarchical database of name servers.

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P2P (peer-to-peer) applications allow participating hosts to act as both client and server, sharing resources/files directly with each other without relying on a central server for the actual data transfer.

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Socket programming provides the application-level API (socket creation, bind, listen, connect, send/receive) used to build networked applications directly atop the transport layer (TCP/UDP).

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An application server hosts and runs application logic, serving requests from client programs (often web browsers) over the network.

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Traffic Analysis Tools Tool Purpose SNMP (Simple Network Management Protocol) Standard protocol for collecting and organizing information about managed network devices MRTG / PRTG Network traffic/performance monitoring tools that graph bandwidth usage and other metrics over time, often using SNMP Packet Tracer A network simulation tool (Cisco) used for designing, configuring, and troubleshooting virtual network topologies Wireshark A packet analyzer/protocol sniffer used to capture and inspect live network traffic in detail, for troubleshooting and analysis

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This final section of the chapter covers types of computer/network security and attacks, cryptography principles including RSA and digital signatures, protocols for securing email/TCP connections/wireless LANs, and firewalls.

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Security Types & Attacks Concept Description Confidentiality Ensures information is accessible only to authorized parties Integrity Ensures information is not altered in an unauthorized manner Availability Ensures authorized users have reliable access to information/resources when needed Passive attack Eavesdropping/traffic monitoring without altering data (e.g. sniffing); hard to detect since data is unaffected Active attack Involves modification of data or system state (e.g. spoofing, replay, denial-of-service, man-in-the- middle) Principles of Cryptography Concept Description Symmetric-key cryptography Same secret key used for both encryption and decryption; fast, but key distribution is a challenge Asymmetric-key (public-key) cryptography Uses a mathematically related key pair: a public key (shared openly) for encryption and a private key (kept secret) for decryption RSA algorithm A widely used asymmetric-key algorithm based on the computational difficulty of factoring the product of two large prime numbers Digital signature Uses a sender's private key to 'sign' data (typically a hash of it); anyone with the sender's public key can verify authenticity and integrity Securing Communications Mechanism Purpose PGP (Pretty Good Privacy) Provides confidentiality and authentication for email using a combination of symmetric and asymmetric cryptography SSL/TLS Secures TCP connections (e.g. for HTTPS), providing encryption, authentication, and integrity for data in transit IPsec Network-layer security framework providing authentication and encryption for IP packets; commonly used to build VPNs VPN (Virtual Private Network) Creates a secure, encrypted tunnel across a public network, extending a private network's reach WEP (Wired Equivalent Privacy) An early, now largely obsolete wireless LAN security protocol with well-known cryptographic weaknesses (superseded by WPA/WPA2/WPA3) Firewalls A firewall monitors and controls incoming/outgoing network traffic based on predetermined security rules, acting as a barrier between a trusted internal network and an untrusted external network.