📡 Common Ports & Protocols
| Protocol |
Port |
Type |
| FTP (Data) |
20 |
TCP |
| FTP (Control) |
21 |
TCP |
| SSH |
22 |
TCP |
| Telnet |
23 |
TCP |
| SMTP |
25 |
TCP |
| DNS |
53 |
TCP/UDP |
| DHCP (Server) |
67 |
UDP |
| DHCP (Client) |
68 |
UDP |
| TFTP |
69 |
UDP |
| HTTP |
80 |
TCP |
| POP3 |
110 |
TCP |
| NTP |
123 |
UDP |
| SNMP (Agent) |
161 |
UDP |
| SNMP (Trap) |
162 |
UDP |
| HTTPS |
443 |
TCP |
| Syslog |
514 |
UDP |
| RDP |
3389 |
TCP |
🔒 Private IP Address Ranges (RFC 1918)
| Class |
Range |
CIDR |
| Class A |
10.0.0.0 - 10.255.255.255 |
10.0.0.0/8 |
| Class B |
172.16.0.0 - 172.31.255.255 |
172.16.0.0/12 |
| Class C |
192.168.0.0 - 192.168.255.255 |
192.168.0.0/16 |
| APIPA |
169.254.0.0 - 169.254.255.255 |
169.254.0.0/16 |
| Loopback |
127.0.0.0 - 127.255.255.255 |
127.0.0.0/8 |
Important: APIPA (169.254.x.x) indicates DHCP failure. Loopback (127.0.0.1) is for local testing.
📶 Special IP Addresses
| Address |
Purpose |
| 0.0.0.0 |
Default route / This network |
| 255.255.255.255 |
Limited broadcast |
| 224.0.0.0 - 239.255.255.255 |
Multicast (Class D) |
| 240.0.0.0 - 255.255.255.255 |
Reserved (Class E) |
🔢 Subnet Mask Quick Reference
| CIDR |
Subnet Mask |
Wildcard Mask |
Total Hosts |
Usable Hosts |
| /24 |
255.255.255.0 |
0.0.0.255 |
256 |
254 |
| /25 |
255.255.255.128 |
0.0.0.127 |
128 |
126 |
| /26 |
255.255.255.192 |
0.0.0.63 |
64 |
62 |
| /27 |
255.255.255.224 |
0.0.0.31 |
32 |
30 |
| /28 |
255.255.255.240 |
0.0.0.15 |
16 |
14 |
| /29 |
255.255.255.248 |
0.0.0.7 |
8 |
6 |
| /30 |
255.255.255.252 |
0.0.0.3 |
4 |
2 |
| /31 |
255.255.255.254 |
0.0.0.1 |
2 |
2* |
| /32 |
255.255.255.255 |
0.0.0.0 |
1 |
1* |
Note: /30 is commonly used for point-to-point links. /31 is used for point-to-point without network/broadcast addresses (RFC 3021).
📊 Administrative Distance (AD)
| Route Source |
AD Value |
| Connected |
0 |
| Static |
1 |
| EIGRP Summary |
5 |
| eBGP |
20 |
| EIGRP (Internal) |
90 |
| IGRP |
100 |
| OSPF |
110 |
| IS-IS |
115 |
| RIP |
120 |
| EIGRP (External) |
170 |
| iBGP |
200 |
Lower is better! Routes with lower AD values are preferred.
⚡ Routing Protocol Metrics
| Protocol |
Metric |
| RIP |
Hop Count (max 15) |
| OSPF |
Cost (based on bandwidth) |
| EIGRP |
Bandwidth + Delay (composite) |
| IS-IS |
Cost (default 10) |
| BGP |
Path Attributes (AS-PATH, etc.) |
⏱️ Routing Protocol Timers
| Protocol |
Hello/Update |
Dead/Hold |
| OSPF (Broadcast) |
10 seconds |
40 seconds |
| OSPF (P2P) |
10 seconds |
40 seconds |
| EIGRP |
5 seconds |
15 seconds |
| RIP |
30 seconds |
180 seconds |
🔌 Cable Types & Specifications
| Cable Type |
Max Speed |
Max Distance |
Use Case |
| Cat5 |
100 Mbps |
100m |
Fast Ethernet |
| Cat5e |
1 Gbps |
100m |
Gigabit Ethernet |
| Cat6 |
10 Gbps |
55m (10Gbps) / 100m (1Gbps) |
10 Gigabit Ethernet (short) |
| Cat6a |
10 Gbps |
100m |
10 Gigabit Ethernet |
| Cat7 |
10 Gbps |
100m |
10 Gigabit Ethernet (shielded) |
| Multimode Fiber (MMF) |
10 Gbps |
550m (10Gbps) |
Campus buildings, data centers |
| Single-mode Fiber (SMF) |
100+ Gbps |
80+ km |
Long distance, WAN links |
📡 Fiber Wavelengths
| Wavelength |
Fiber Type |
Typical Distance |
| 850nm |
Multimode |
Up to 550m |
| 1310nm |
Single-mode |
Up to 10km |
| 1550nm |
Single-mode |
Up to 80+ km |
📚 OSI Model Layers
| # |
Layer |
PDU |
Key Protocols/Devices |
| 7 |
Application |
Data |
HTTP, FTP, DNS, SMTP |
| 6 |
Presentation |
Data |
SSL/TLS, JPEG, ASCII |
| 5 |
Session |
Data |
NetBIOS, RPC |
| 4 |
Transport |
Segment |
TCP, UDP |
| 3 |
Network |
Packet |
IP, ICMP, Router |
| 2 |
Data Link |
Frame |
Ethernet, Switch, MAC |
| 1 |
Physical |
Bits |
Cables, Hub, Signals |
Mnemonic: Please Do Not Throw Sausage Pizza Away
(Physical, Data Link, Network, Transport, Session, Presentation, Application)
🔄 TCP vs UDP Comparison
| Feature |
TCP |
UDP |
| Connection |
Connection-oriented |
Connectionless |
| Reliability |
Reliable |
Unreliable |
| Ordering |
Ordered delivery |
No ordering |
| Speed |
Slower |
Faster |
| Overhead |
Higher |
Lower |
| Error Checking |
Extensive |
Basic checksum |
| Use Cases |
HTTP, FTP, Email |
DNS, DHCP, VoIP, Video |
🌲 STP Port States
| State |
Learns MAC? |
Forwards? |
Duration |
| Disabled |
No |
No |
- |
| Blocking |
No |
No |
20 sec |
| Listening |
No |
No |
15 sec |
| Learning |
Yes |
No |
15 sec |
| Forwarding |
Yes |
Yes |
- |
Total Convergence: 50 seconds (20+15+15) for traditional STP