📘 Key Concepts — Quick Reference
▾OSI Model (7 Layers)
Top to bottom, each layer has its own protocol data unit (PDU):
| # | Layer | PDU | Examples |
|---|---|---|---|
| 7 | Application | Data | HTTP, FTP, SMTP, DNS |
| 6 | Presentation | Data | Encryption, compression, formatting |
| 5 | Session | Data | Session setup/teardown (e.g. NetBIOS) |
| 4 | Transport | Segment (TCP) / Datagram (UDP) | TCP, UDP |
| 3 | Network | Packet | IP, ICMP, routers |
| 2 | Data Link | Frame | Ethernet, switches, MAC addresses |
| 1 | Physical | Bits | Cables, radio signals, hubs |
TCP/IP Model (4 Layers) vs OSI
| TCP/IP Layer | Maps to OSI |
|---|---|
| Application | Application, Presentation, Session |
| Transport | Transport |
| Internet | Network |
| Network Access | Data Link, Physical |
Common Protocols & Default Ports
DNS · 53
DHCP · 67/68
HTTP · 80
HTTPS · 443
FTP · 20/21
SSH · 22
Telnet · 23
SMTP · 25
POP3 · 110
IMAP · 143
SNMP · 161
Networking Operating Systems (NOS)
Software that manages network resources: user accounts, shared files/printers, security, and device communication. Examples: Windows Server, Linux, Cisco IOS (runs on routers/switches).
Communication Modes
| Mode | Description |
|---|---|
| Simplex | One direction only (e.g. radio broadcast) |
| Half-duplex | Both directions, one at a time (e.g. walkie-talkie) |
| Full-duplex | Both directions simultaneously (e.g. phone call) |
Segmentation, Fragmentation, Replication & Reassembly
Segmentation: the Transport layer breaks large data into smaller segments, each tagged with a sequence number.
Fragmentation: if a packet exceeds a network's maximum transmission unit (MTU), the Network layer splits it into smaller fragments.
Reassembly: the destination uses sequence numbers to reorder segments/fragments back into the original data.
Replication: copying data across multiple servers or locations to improve availability and fault tolerance.