OSI Model
A seven-layer conceptual framework developed by ISO that describes how communications should occur in a computer network from physical transmission to application interfaces
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| Term | Definition |
|---|---|
| OSI Model | A seven-layer conceptual framework developed by ISO that describes how communications should occur in a computer network from physical transmission to application interfaces |
| Physical Layer (Layer 1) | Defines the physical medium and electrical/optical/radio signals used to transmit raw binary data (examples: Ethernet cables WiFi radio bands 2.4GHz/5GHz/6GHz) |
| Data Link Layer (Layer 2) | Defines protocols for reliable data transfer between adjacent nodes on the same network segment using MAC addresses in frames (examples: Ethernet 802.3 WiFi 802.11) |
| Network Layer (Layer 3) | Handles logical addressing and routing of packets between different networks using IP addresses (examples: IP ICMP IPSec) |
| Transport Layer (Layer 4) | Provides end-to-end communication between applications using port numbers and handles segmentation and reliability (examples: TCP provides reliability UDP does not) |
| Session Layer (Layer 5) | Establishes maintains and terminates communication sessions between applications (examples: NFS RPC) |
| Presentation Layer (Layer 6) | Handles data format translation encryption and compression to ensure data is readable by the application layer (examples: Unicode MIME JPEG PNG) |
| Application Layer (Layer 7) | Provides network services directly to end-user applications (examples: HTTP for web FTP for file transfer DNS for name resolution SMTP for email) |
| MAC Address | Media Access Control address - a 48-bit (6-byte) hardware address in hexadecimal format (xx:xx:xx:xx:xx:xx) burned into network interface cards for Layer 2 identification |
| Network Segment | A portion of a network where all devices share the same broadcast domain and can communicate at Layer 2 without routing (example: all computers connected to one switch) |
| Broadcast Domain | A network segment where a broadcast sent by one device reaches all other devices - routers separate broadcast domains while switches do not |
| RFC 1918 Private IP Ranges | Three reserved IPv4 address ranges for private networks: 10.0.0.0/8 (16.7M addresses) 172.16.0.0/12 (1M addresses) 192.168.0.0/16 (65K addresses) |
| DHCP | Dynamic Host Configuration Protocol - automatically assigns IP configuration (IP address subnet mask default gateway DNS servers) to clients using UDP ports 67 (server) and 68 (client) |
| DHCP DORA Process | Four-step DHCP process: Discover (client broadcasts request) Offer (server unicasts available IP) Request (client broadcasts acceptance) Acknowledge (server unicasts confirmation and lease time) |
| ARP | Address Resolution Protocol - maps IP addresses to MAC addresses on local network by broadcasting request to ff:ff:ff:ff:ff:ff and receiving unicast reply - results cached in ARP table to avoid repeated broadcasts |
| ARP Cache | A table maintained by each host storing recent IP-to-MAC address mappings with expiration times to reduce network broadcasts and improve efficiency |
| ICMP | Internet Control Message Protocol - Layer 3 protocol for network diagnostics and error reporting (examples: ping uses Echo Request/Reply traceroute uses TTL exceeded messages) |
| TTL | Time To Live - an 8-bit field in IP packet header representing maximum hop count (not seconds) - decremented by 1 at each router and packet dropped when reaching 0 with ICMP Time Exceeded sent to source |
| Subnet Mask | A 32-bit number that defines which portion of an IP address represents the network and which portion represents the host (example: 255.255.255.0 or /24) |
| Default Gateway | The router IP address that a host uses to send packets destined for networks outside its local subnet - typically the router interface on the local network |
| OSPF | Open Shortest Path First - a link-state routing protocol where each router maintains complete network topology map and uses Dijkstra algorithm to calculate shortest paths |
| EIGRP | Enhanced Interior Gateway Routing Protocol - Cisco proprietary advanced distance-vector (hybrid) protocol using bandwidth and delay metrics to calculate best routes |
| RIP | Routing Information Protocol - a simple distance-vector protocol for small networks using hop count as metric (max 15 hops) where routers share routing tables with neighbors every 30 seconds |
| BGP | Border Gateway Protocol - the routing protocol of the internet used between autonomous systems (AS) - uses path vector algorithm and policy-based routing rather than metrics |
| NAT | Network Address Translation - translates private IP addresses to public IP addresses allowing multiple internal hosts to share one public IP - NAT router maintains translation table tracking connections |
| PAT | Port Address Translation (also called NAT overload) - type of NAT that uses port numbers in addition to IP addresses to allow many internal hosts to share a single public IP address |
| TCP | Transmission Control Protocol - Layer 4 connection-oriented protocol providing reliable ordered delivery with error checking flow control and congestion control (used for HTTP FTP SSH) |
| UDP | User Datagram Protocol - Layer 4 connectionless protocol providing fast unreliable delivery without error correction or flow control (used for DNS DHCP VoIP streaming) |
| Port Number | A 16-bit number (0-65535) used at Layer 4 to identify specific applications or services - well-known ports (0-1023) reserved for common services like HTTP (80) HTTPS (443) SSH (22) |
| DNS | Domain Name System - translates human-readable domain names into IP addresses using UDP port 53 for queries (TCP for zone transfers) - hierarchical distributed database system |
| List All The Layers In the OSI Model | Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer |