| CIDR | Subnet Mask | Host Bits | Hosts |
|---|---|---|---|
| /32 | 255.255.255.255 | 0 | 1 (Host) |
| /31 | 255.255.255.254 | 1 | 0 (P2P) |
| /30 | 255.255.255.252 | 2 | 2 |
| /29 | 255.255.255.248 | 3 | 6 |
| /28 | 255.255.255.240 | 4 | 14 |
| /27 | 255.255.255.224 | 5 | 30 |
| /26 | 255.255.255.192 | 6 | 62 |
| /25 | 255.255.255.128 | 7 | 126 |
| /24 | 255.255.255.0 | 8 | 254 |
| /23 | 255.255.254.0 | 9 | 510 |
| /22 | 255.255.252.0 | 10 | 1,022 |
| /21 | 255.255.248.0 | 11 | 2,046 |
| /20 | 255.255.240.0 | 12 | 4,094 |
| /19 | 255.255.224.0 | 13 | 8,190 |
| /18 | 255.255.192.0 | 14 | 16,382 |
| /17 | 255.255.128.0 | 15 | 32,766 |
| /16 | 255.255.0.0 | 16 | 65,534 |
| /15 | 255.254.0.0 | 17 | 131,070 |
| /14 | 255.252.0.0 | 18 | 262,142 |
| /13 | 255.248.0.0 | 19 | 524,286 |
| /12 | 255.240.0.0 | 20 | 1,048,574 |
| /11 | 255.224.0.0 | 21 | 2,097,150 |
| /10 | 255.192.0.0 | 22 | 4,194,302 |
| /9 | 255.128.0.0 | 23 | 8,388,606 |
| /8 | 255.0.0.0 | 24 | 16,777,214 |
| Port | Protocol | Type | Description |
|---|---|---|---|
| 20, 21 | FTP | TCP | File Transfer (Data: 20, Control: 21) |
| 22 | SSH | TCP | Secure Shell (Encrypted CLI) |
| 23 | Telnet | TCP | Unencrypted Remote CLI |
| 25 | SMTP | TCP | Simple Mail Transfer Protocol |
| 53 | DNS | TCP/UDP | Domain Name System |
| 67, 68 | DHCP | UDP | Server (67), Client (68) |
| 69 | TFTP | UDP | Trivial File Transfer Protocol |
| 80 | HTTP | TCP | Hypertext Transfer Protocol |
| 110 | POP3 | TCP | Post Office Protocol v3 |
| 123 | NTP | UDP | Network Time Protocol |
| 143 | IMAP | TCP | Internet Message Access Protocol |
| 161, 162 | SNMP | UDP | Agent (161), Traps (162) |
| 179 | BGP | TCP | Border Gateway Protocol |
| 443 | HTTPS | TCP | HTTP Secure (TLS/SSL) |
| 514 | Syslog | UDP | System Logging Protocol |
| 1812, 1813 | RADIUS | UDP | Auth (1812), Accounting (1813) |
| Route Source | AD Value | Priority |
|---|---|---|
| Connected Interface | 0 | Highest |
| Static Route | 1 | Default Static |
| EIGRP Summary Route | 5 | Internal Summary |
| External BGP (eBGP) | 20 | Between AS |
| Internal EIGRP | 90 | Cisco Proprietary |
| OSPF | 110 | Open Shortest Path First |
| IS-IS | 115 | Intermediate System |
| RIP | 120 | Routing Info Protocol |
| External EIGRP | 170 | Redistributed EIGRP |
| Internal BGP (iBGP) | 200 | Within AS |
| DHCP Default Route | 254 | Assigned via DHCP |
| Unusable / Untrusted | 255 | Discarded |
| Prefix / Address | Type | Description / Scope |
|---|---|---|
| ::/128 | Unspecified | Used when no IP is assigned |
| ::1/128 | Loopback | Local Host Loopback |
| 2000::/3 | GUA | Global Unicast (Public) |
| fe80::/10 | Link-Local | Auto-configured on Interface |
| fc00::/7 | ULA | Unique Local (Private IPv6) |
| ff00::/8 | Multicast | All IPv6 Multicast Addresses |
| ff02::1 | Multicast | All IPv6 Nodes (Link-Local) |
| ff02::2 | Multicast | All IPv6 Routers (Link-Local) |
| ff02::5 | Multicast | All OSPFv3 Routers |
| ff02::6 | Multicast | OSPFv3 Designated Routers |
| ff02::9 | Multicast | All RIPng Routers |
| ff02::a | Multicast | All EIGRPv6 Routers |
| Component | Volatility | Contents |
|---|---|---|
| RAM | Volatile | Running-config, ARP table, Routing table |
| NVRAM | Non-Volatile | Startup-config, Config-register |
| Flash | Non-Volatile | IOS image (.bin), vlan.dat file |
| ROM | Non-Volatile | POST, Bootstrap, ROMmon mode |
| 0x2102 | Config Reg | Normal boot (Flash IOS + NVRAM config) |
| 0x2142 | Config Reg | Bypass NVRAM config (Password recovery) |
| EtherType 0x0800 | Frame Field | IPv4 Packet Payload |
| EtherType 0x0806 | Frame Field | ARP Frame Payload |
| EtherType 0x86DD | Frame Field | IPv6 Packet Payload |
| EtherType 0x8100 | Frame Field | 802.1Q VLAN Tagged Frame |