Networking fundamentals
Why it matters: Many Security+ questions assume you know the OSI model, TCP/IP basics and IP addressing. Your guide uses these ideas without teaching them from scratch.
Overview
Supplementary — not from your PDFNetworks are easier to secure when you know how traffic flows. The OSI model is a seven-layer way to describe that flow. TCP/IP is the protocol suite actually used, and IP addressing and subnetting decide who can reach whom.
OSI model
| Layer | Name | Examples | Security relevance |
|---|---|---|---|
| 7 | Application | HTTP, DNS, SMTP | WAFs and layer 7 firewalls inspect here |
| 6 | Presentation | Encoding, TLS formatting | Data formats, encryption presentation |
| 5 | Session | Session setup and teardown | Session hijacking concerns |
| 4 | Transport | TCP, UDP, ports | Layer 4 firewalls filter on ports |
| 3 | Network | IP, ICMP, routing | ACLs on routers, IPsec |
| 2 | Data link | Ethernet, MAC addresses, switches, VLANs | Port security, 802.1X, ARP issues |
| 1 | Physical | Cables, radio | Physical access, wiretaps, jamming |
TCP vs UDP
- TCP is connection-oriented and reliable. It sets up with a three-way handshake: SYN → SYN/ACK → ACK. Used by web, email and SSH.
- UDP is connectionless and has no delivery guarantee, but low overhead. Used by DNS queries, VoIP, streaming and DHCP.
- Firewall rules and scanners care about the difference: a UDP 'no reply' is ambiguous, while TCP gives clear answers.
IP addressing essentials
- IPv4 private ranges (not routed on the internet):
10.0.0.0/8,172.16.0.0/12,192.168.0.0/16. 127.0.0.0/8is loopback, and169.254.0.0/16(APIPA) means a device didn't get a DHCP address.- CIDR:
/24= 256 addresses (254 usable hosts),/25= 128,/26= 64,/30= 4 (2 usable). - DHCP hands out addresses (Discover, Offer, Request, Acknowledge). DNS turns names into addresses. ARP maps IP to MAC on the local network.
- IPv6 addresses are 128-bit;
::1is loopback andfe80::/10is link-local.