Master the building blocks of networking - OSI model, TCP/IP, addressing, and subnetting
The OSI (Open Systems Interconnection) model is a conceptual framework that standardizes network communication into 7 distinct layers. Understanding this model is fundamental to troubleshooting and designing networks.
The OSI model provides a common language for networking professionals. When troubleshooting, you can isolate problems to specific layers, making resolution faster and more efficient.
Memorize the layers and their PDUs! Common mnemonics:
| Layer | Name | PDU | Function | Devices |
|---|---|---|---|---|
| 7 | Application | Data | Network services to applications | Gateways, Firewalls |
| 6 | Presentation | Data | Data translation, encryption | Gateways |
| 5 | Session | Data | Session management | Gateways |
| 4 | Transport | Segment | End-to-end delivery | Gateways, Firewalls |
| 3 | Network | Packet | Logical addressing, routing | Routers, L3 Switches |
| 2 | Data Link | Frame | Physical addressing, switching | Switches, Bridges |
| 1 | Physical | Bits | Bit transmission | Hubs, Cables, NICs |
The TCP/IP model is the practical implementation model used on the internet and modern networks. It has 4 layers that map to the OSI model.
| Feature | TCP | UDP |
|---|---|---|
| Connection | Connection-oriented | Connectionless |
| Reliability | Guaranteed delivery | Best-effort delivery |
| Ordering | Ordered delivery | No ordering |
| Flow Control | Yes (windowing) | No |
| Speed | Slower (overhead) | Faster |
| Header Size | 20-60 bytes | 8 bytes |
| Use Cases | HTTP, FTP, SSH, Email | DNS, DHCP, VoIP, Streaming |
Connection termination uses a four-way handshake: FIN, ACK, FIN, ACK
| Port | Protocol | Service | Description |
|---|---|---|---|
| 20 | TCP | FTP Data | File transfer data |
| 21 | TCP | FTP Control | File transfer commands |
| 22 | TCP | SSH | Secure Shell |
| 23 | TCP | Telnet | Remote terminal (insecure) |
| 25 | TCP | SMTP | Email sending |
| 53 | TCP/UDP | DNS | Domain Name System |
| 67/68 | UDP | DHCP | Dynamic IP assignment |
| 80 | TCP | HTTP | Web traffic |
| 110 | TCP | POP3 | Email retrieval |
| 143 | TCP | IMAP | Email management |
| 161/162 | UDP | SNMP | Network monitoring |
| 443 | TCP | HTTPS | Secure web traffic |
| 514 | UDP | Syslog | System logging |
Know the port ranges:
IPv4 addresses are 32-bit numbers written in dotted decimal notation (e.g., 192.168.1.1). Understanding IP addressing is crucial for network design and troubleshooting.
| Class | First Octet | Default Mask | Networks | Hosts/Network | Purpose |
|---|---|---|---|---|---|
| A | 1-126 | 255.0.0.0 (/8) | 126 | 16,777,214 | Large organizations |
| B | 128-191 | 255.255.0.0 (/16) | 16,384 | 65,534 | Medium organizations |
| C | 192-223 | 255.255.255.0 (/24) | 2,097,152 | 254 | Small organizations |
| D | 224-239 | N/A | N/A | N/A | Multicast |
| E | 240-255 | N/A | N/A | N/A | Experimental |
These addresses are not routable on the internet and can be reused in any private network:
| Address | Purpose | Description |
|---|---|---|
| 0.0.0.0 | Default Route | Represents all networks (used in routing) |
| 127.0.0.0/8 | Loopback | Testing local TCP/IP stack (127.0.0.1) |
| 169.254.0.0/16 | APIPA | Auto-assigned when DHCP fails |
| 255.255.255.255 | Limited Broadcast | Broadcast to all hosts on local network |
Subnetting divides a network into smaller subnetworks. This improves security, reduces broadcast traffic, and makes efficient use of IP addresses.
| CIDR | Subnet Mask | Total IPs | Usable Hosts | Block Size |
|---|---|---|---|---|
| /24 | 255.255.255.0 | 256 | 254 | 256 |
| /25 | 255.255.255.128 | 128 | 126 | 128 |
| /26 | 255.255.255.192 | 64 | 62 | 64 |
| /27 | 255.255.255.224 | 32 | 30 | 32 |
| /28 | 255.255.255.240 | 16 | 14 | 16 |
| /29 | 255.255.255.248 | 8 | 6 | 8 |
| /30 | 255.255.255.252 | 4 | 2 | 4 |
| /31 | 255.255.255.254 | 2 | 2* | 2 |
| /32 | 255.255.255.255 | 1 | 1* | 1 |
Step 1: /26 means 255.255.255.192
Step 2: Block size = 256 - 192 = 64
Step 3: Network boundaries: 0, 64, 128, 192
Step 4: 67 falls between 64 and 128
Results:
Practice subnetting until you can do it quickly! The exam has multiple subnetting questions and you need to be fast. Remember:
IPv6 uses 128-bit addresses written in hexadecimal, providing virtually unlimited addresses. It was created to solve IPv4 address exhaustion.
| Type | Prefix | Description |
|---|---|---|
| Global Unicast | 2000::/3 | Routable on internet (like public IPv4) |
| Link-Local | FE80::/10 | Auto-configured, not routable |
| Unique Local | FC00::/7 | Private addresses (like RFC 1918) |
| Multicast | FF00::/8 | One-to-many communication |
| Loopback | ::1 | Localhost |
| Unspecified | :: | All zeros (like 0.0.0.0) |
Ethernet is the most common LAN technology, operating at Layer 1 and Layer 2 of the OSI model.
MAC addresses are 48-bit hardware addresses burned into NICs:
AA:BB:CC:DD:EE:FF or AA-BB-CC-DD-EE-FF
| Standard | Speed | Cable Type | Max Distance |
|---|---|---|---|
| 10BASE-T | 10 Mbps | Cat 3 UTP | 100m |
| 100BASE-TX | 100 Mbps | Cat 5 UTP | 100m |
| 1000BASE-T | 1 Gbps | Cat 5e/6 UTP | 100m |
| 10GBASE-T | 10 Gbps | Cat 6a/7 UTP | 100m |
| 1000BASE-SX | 1 Gbps | Multimode Fiber | 550m |
| 1000BASE-LX | 1 Gbps | Single-mode Fiber | 5km |
| Device | OSI Layer | Addressing | Function |
|---|---|---|---|
| Hub | Layer 1 | None | Repeats signals to all ports (obsolete) |
| Switch | Layer 2 | MAC addresses | Forwards frames based on MAC table |
| Router | Layer 3 | IP addresses | Routes packets between networks |
| L3 Switch | Layer 2-3 | MAC + IP | Switching + routing capabilities |
| Firewall | Layer 3-7 | IP + Ports | Filters traffic based on rules |
| Category | Max Speed | Max Bandwidth | Use Case |
|---|---|---|---|
| Cat 5 | 100 Mbps | 100 MHz | Fast Ethernet |
| Cat 5e | 1 Gbps | 100 MHz | Gigabit Ethernet |
| Cat 6 | 10 Gbps (55m) | 250 MHz | 10G short runs |
| Cat 6a | 10 Gbps | 500 MHz | 10G Ethernet |
| Cat 7 | 10 Gbps | 600 MHz | Data centers |
Both ends same wiring (T568A or T568B)
Use: Different device types
Different wiring on each end
Use: Same device types
Modern devices with Auto-MDIX can automatically detect and adjust for cable type. However, you should still know when to use straight-through vs crossover cables for the exam!