# CCNA Subnetting Cheat Sheet **VantagePoint Networks -- Study Guide Series** --- | Field | Value | |--------------------|--------------------------------------------| | Document ID | VPN-SG-CCNA-001 | | Version | 1.0 | | Author | | | Target Exam | Cisco CCNA 200-301 | | Effective Date | | | Organisation | | --- ## Table of Contents 1. [Complete CIDR Reference Table](#1-complete-cidr-reference-table) 2. [Powers of 2 Table](#2-powers-of-2-table) 3. [The Magic Number Method](#3-the-magic-number-method) 4. [Worked Examples](#4-worked-examples) 5. [Private IP Address Ranges (RFC 1918)](#5-private-ip-address-ranges-rfc-1918) 6. [Common Port Numbers](#6-common-port-numbers) 7. [IPv4 Header Diagram](#7-ipv4-header-diagram) 8. [Routing Protocol Reference](#8-routing-protocol-reference) 9. [Spanning Tree Protocol Reference](#9-spanning-tree-protocol-reference) 10. [Quick Reference Summary](#10-quick-reference-summary) --- ## 1. Complete CIDR Reference Table ### CIDR Notation, Subnet Masks, and Host Counts (/0 through /32) | CIDR | Subnet Mask | Wildcard Mask | Total Addresses | Usable Hosts | Class | |-------|---------------------|---------------------|----------------------|----------------------|-----------| | /0 | 0.0.0.0 | 255.255.255.255 | 4,294,967,296 | 4,294,967,294 | Default | | /1 | 128.0.0.0 | 127.255.255.255 | 2,147,483,648 | 2,147,483,646 | | | /2 | 192.0.0.0 | 63.255.255.255 | 1,073,741,824 | 1,073,741,822 | | | /3 | 224.0.0.0 | 31.255.255.255 | 536,870,912 | 536,870,910 | | | /4 | 240.0.0.0 | 15.255.255.255 | 268,435,456 | 268,435,454 | | | /5 | 248.0.0.0 | 7.255.255.255 | 134,217,728 | 134,217,726 | | | /6 | 252.0.0.0 | 3.255.255.255 | 67,108,864 | 67,108,862 | | | /7 | 254.0.0.0 | 1.255.255.255 | 33,554,432 | 33,554,430 | | | /8 | 255.0.0.0 | 0.255.255.255 | 16,777,216 | 16,777,214 | Class A | | /9 | 255.128.0.0 | 0.127.255.255 | 8,388,608 | 8,388,606 | | | /10 | 255.192.0.0 | 0.63.255.255 | 4,194,304 | 4,194,302 | | | /11 | 255.224.0.0 | 0.31.255.255 | 2,097,152 | 2,097,150 | | | /12 | 255.240.0.0 | 0.15.255.255 | 1,048,576 | 1,048,574 | | | /13 | 255.248.0.0 | 0.7.255.255 | 524,288 | 524,286 | | | /14 | 255.252.0.0 | 0.3.255.255 | 262,144 | 262,142 | | | /15 | 255.254.0.0 | 0.1.255.255 | 131,072 | 131,070 | | | /16 | 255.255.0.0 | 0.0.255.255 | 65,536 | 65,534 | Class B | | /17 | 255.255.128.0 | 0.0.127.255 | 32,768 | 32,766 | | | /18 | 255.255.192.0 | 0.0.63.255 | 16,384 | 16,382 | | | /19 | 255.255.224.0 | 0.0.31.255 | 8,192 | 8,190 | | | /20 | 255.255.240.0 | 0.0.15.255 | 4,096 | 4,094 | | | /21 | 255.255.248.0 | 0.0.7.255 | 2,048 | 2,046 | | | /22 | 255.255.252.0 | 0.0.3.255 | 1,024 | 1,022 | | | /23 | 255.255.254.0 | 0.0.1.255 | 512 | 510 | | | /24 | 255.255.255.0 | 0.0.0.255 | 256 | 254 | Class C | | /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 | P2P Link | | /31 | 255.255.255.254 | 0.0.0.1 | 2 | 2 (RFC 3021) | P2P Link | | /32 | 255.255.255.255 | 0.0.0.0 | 1 | 1 (host route) | Host | **Key formulas:** - Total addresses = 2^(32 - prefix length) - Usable hosts = 2^(32 - prefix length) - 2 (subtract network and broadcast) - Wildcard mask = 255.255.255.255 - subnet mask --- ## 2. Powers of 2 Table This table is essential for rapid subnetting calculations. | Exponent | Value | Common Use | |----------|--------------|-------------------------------------------| | 2^0 | 1 | /32 host route | | 2^1 | 2 | /31 point-to-point link (RFC 3021) | | 2^2 | 4 | /30 point-to-point link (2 usable) | | 2^3 | 8 | /29 smallest practical subnet (6 usable) | | 2^4 | 16 | /28 small subnet (14 usable) | | 2^5 | 32 | /27 subnet (30 usable) | | 2^6 | 64 | /26 subnet (62 usable) | | 2^7 | 128 | /25 subnet (126 usable) | | 2^8 | 256 | /24 Class C equivalent (254 usable) | | 2^9 | 512 | /23 (510 usable) | | 2^10 | 1,024 | /22 (1,022 usable) | | 2^11 | 2,048 | /21 (2,046 usable) | | 2^12 | 4,096 | /20 (4,094 usable) | | 2^13 | 8,192 | /19 (8,190 usable) | | 2^14 | 16,384 | /18 (16,382 usable) | | 2^15 | 32,768 | /17 (32,766 usable) | | 2^16 | 65,536 | /16 Class B equivalent (65,534 usable) | | 2^17 | 131,072 | | | 2^18 | 262,144 | | | 2^19 | 524,288 | | | 2^20 | 1,048,576 | /12 (1,048,574 usable) | | 2^21 | 2,097,152 | | | 2^22 | 4,194,304 | | | 2^23 | 8,388,608 | | | 2^24 | 16,777,216 | /8 Class A equivalent (16,777,214 usable) | **Memorisation tip:** Each step doubles. Start from 2^10 = 1,024 and work up or down. --- ## 3. The Magic Number Method The Magic Number Method is the fastest way to calculate subnet boundaries without binary conversion. ### How It Works 1. **Identify the interesting octet** -- the octet where the subnet mask is NOT 0 and NOT 255 2. **Calculate the magic number** -- subtract the interesting octet value from 256 3. **Multiples of the magic number** define the subnet boundaries in that octet ### Formula ``` Magic Number = 256 - (subnet mask value in the interesting octet) ``` ### Step-by-Step Process 1. Write the subnet mask 2. Find the interesting octet (the one that is not 0 or 255) 3. Magic Number = 256 - interesting octet value 4. Network addresses are multiples of the magic number in that octet 5. Broadcast address = next network address - 1 6. First usable host = network address + 1 7. Last usable host = broadcast address - 1 --- ## 4. Worked Examples ### Example 1: Find the subnet for 192.168.1.130/26 **Given:** IP = 192.168.1.130, Mask = /26 = 255.255.255.192 **Step 1:** Interesting octet = 4th octet (192) **Step 2:** Magic Number = 256 - 192 = 64 **Step 3:** List subnet boundaries in the 4th octet: 0, 64, 128, 192 **Step 4:** 130 falls between 128 and 192 **Result:** - Network address: 192.168.1.128 - First usable host: 192.168.1.129 - Last usable host: 192.168.1.191 - Broadcast address: 192.168.1.191 - Usable hosts: 62 Wait -- let me correct that: - Broadcast address: 192.168.1.191 (next subnet 192 minus 1) - Last usable host: 192.168.1.190 **Verification:** 192 - 128 = 64 addresses. 64 - 2 = 62 usable. Correct. --- ### Example 2: Find the subnet for 10.45.67.200/21 **Given:** IP = 10.45.67.200, Mask = /21 = 255.255.248.0 **Step 1:** Interesting octet = 3rd octet (248) **Step 2:** Magic Number = 256 - 248 = 8 **Step 3:** List subnet boundaries in the 3rd octet: 0, 8, 16, 24, 32, 40, 48, 56, 64, 72... **Step 4:** 67 falls between 64 and 72 **Result:** - Network address: 10.45.64.0 - First usable host: 10.45.64.1 - Last usable host: 10.45.71.254 - Broadcast address: 10.45.71.255 - Usable hosts: 2,046 **Verification:** 8 x 256 = 2,048 addresses. 2,048 - 2 = 2,046 usable. Correct. --- ### Example 3: How many /28 subnets fit in 172.16.5.0/24? **Given:** Parent network = /24, subnets needed = /28 **Step 1:** Difference in prefix = 28 - 24 = 4 bits borrowed **Step 2:** Number of subnets = 2^4 = 16 **Step 3:** Hosts per subnet = 2^(32-28) - 2 = 16 - 2 = 14 **Result:** - 16 subnets, each with 14 usable hosts - Subnet boundaries (4th octet): 0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240 | Subnet # | Network Address | First Host | Last Host | Broadcast | |----------|------------------|------------------|------------------|------------------| | 1 | 172.16.5.0 | 172.16.5.1 | 172.16.5.14 | 172.16.5.15 | | 2 | 172.16.5.16 | 172.16.5.17 | 172.16.5.30 | 172.16.5.31 | | 3 | 172.16.5.32 | 172.16.5.33 | 172.16.5.46 | 172.16.5.47 | | 4 | 172.16.5.48 | 172.16.5.49 | 172.16.5.62 | 172.16.5.63 | | ... | ... | ... | ... | ... | | 16 | 172.16.5.240 | 172.16.5.241 | 172.16.5.254 | 172.16.5.255 | --- ### Example 4: Design subnets for 192.168.10.0/24 with the following requirements | Department | Hosts Needed | |-------------|-------------| | Engineering | 50 | | Sales | 25 | | Management | 10 | | Server VLAN | 5 | | P2P Links | 2 links | **Rule:** Always allocate the largest subnet first. **Step 1: Engineering (50 hosts)** - Need 50 hosts. Next power of 2 >= 52 (hosts + network + broadcast) = 64 = 2^6 - Prefix = 32 - 6 = /26 (62 usable hosts) - Subnet: 192.168.10.0/26 (range .0 to .63) **Step 2: Sales (25 hosts)** - Need 25 hosts. Next power of 2 >= 27 = 32 = 2^5 - Prefix = 32 - 5 = /27 (30 usable hosts) - Subnet: 192.168.10.64/27 (range .64 to .95) **Step 3: Management (10 hosts)** - Need 10 hosts. Next power of 2 >= 12 = 16 = 2^4 - Prefix = 32 - 4 = /28 (14 usable hosts) - Subnet: 192.168.10.96/28 (range .96 to .111) **Step 4: Server VLAN (5 hosts)** - Need 5 hosts. Next power of 2 >= 7 = 8 = 2^3 - Prefix = 32 - 3 = /29 (6 usable hosts) - Subnet: 192.168.10.112/29 (range .112 to .119) **Step 5: P2P Links (2 links, 2 hosts each)** - /30 per link (2 usable hosts each) - Link 1: 192.168.10.120/30 (range .120 to .123) - Link 2: 192.168.10.124/30 (range .124 to .127) **Summary:** | Department | Subnet | Range | Usable Hosts | Allocated | |-------------|---------------------|--------------------|-------------|-----------| | Engineering | 192.168.10.0/26 | .0 - .63 | 62 | 50 | | Sales | 192.168.10.64/27 | .64 - .95 | 30 | 25 | | Management | 192.168.10.96/28 | .96 - .111 | 14 | 10 | | Server VLAN | 192.168.10.112/29 | .112 - .119 | 6 | 5 | | P2P Link 1 | 192.168.10.120/30 | .120 - .123 | 2 | 2 | | P2P Link 2 | 192.168.10.124/30 | .124 - .127 | 2 | 2 | | Remaining | 192.168.10.128/25 | .128 - .255 | 126 | - | Remaining address space: 192.168.10.128 to 192.168.10.255 (/25) available for future use. --- ### Example 5: Given 10.0.0.0/8, what is the subnet mask for 500 subnets? **Step 1:** Find the number of bits needed for 500 subnets - 2^9 = 512 (this is the smallest power of 2 that is >= 500) - Bits to borrow = 9 **Step 2:** New prefix = 8 + 9 = /17 **Step 3:** Subnet mask for /17 = 255.255.128.0 **Step 4:** Hosts per subnet = 2^(32-17) - 2 = 2^15 - 2 = 32,766 **Result:** - 512 subnets available (500 required, 12 spare) - Each subnet supports 32,766 hosts - Subnet mask: 255.255.128.0 (/17) --- ## 5. Private IP Address Ranges (RFC 1918) ### RFC 1918 Private Address Space | Class | Range | CIDR Notation | Subnet Mask | Total Addresses | Subnets Available | |---------|-------------------------------|-----------------|-----------------|-----------------|----------------------| | Class A | 10.0.0.0 - 10.255.255.255 | 10.0.0.0/8 | 255.0.0.0 | 16,777,216 | 1 Class A network | | Class B | 172.16.0.0 - 172.31.255.255 | 172.16.0.0/12 | 255.240.0.0 | 1,048,576 | 16 Class B networks | | Class C | 192.168.0.0 - 192.168.255.255 | 192.168.0.0/16 | 255.255.0.0 | 65,536 | 256 Class C networks | ### Other Special-Use Addresses | Range | Purpose | Reference | |---------------------------------|--------------------------------------|-----------------| | 0.0.0.0/8 | "This" network | RFC 791 | | 127.0.0.0/8 | Loopback | RFC 1122 | | 169.254.0.0/16 | Link-local (APIPA) | RFC 3927 | | 192.0.2.0/24 | Documentation (TEST-NET-1) | RFC 5737 | | 198.51.100.0/24 | Documentation (TEST-NET-2) | RFC 5737 | | 203.0.113.0/24 | Documentation (TEST-NET-3) | RFC 5737 | | 198.18.0.0/15 | Benchmarking | RFC 2544 | | 100.64.0.0/10 | Carrier-grade NAT (CGN/CGNAT) | RFC 6598 | | 224.0.0.0/4 | Multicast | RFC 5771 | | 240.0.0.0/4 | Reserved (formerly Class E) | RFC 1112 | | 255.255.255.255/32 | Limited broadcast | RFC 919 | --- ## 6. Common Port Numbers ### Top 30 Ports for CCNA | Port | Protocol | Service | Description | |---------|----------|--------------------------------|------------------------------------------| | 20 | TCP | FTP Data | File Transfer Protocol (data channel) | | 21 | TCP | FTP Control | File Transfer Protocol (control channel) | | 22 | TCP | SSH | Secure Shell (remote access, SFTP, SCP) | | 23 | TCP | Telnet | Unencrypted remote terminal access | | 25 | TCP | SMTP | Simple Mail Transfer Protocol | | 53 | TCP/UDP | DNS | Domain Name System | | 67 | UDP | DHCP Server | Dynamic Host Configuration Protocol | | 68 | UDP | DHCP Client | DHCP client responses | | 69 | UDP | TFTP | Trivial File Transfer Protocol | | 80 | TCP | HTTP | Hypertext Transfer Protocol | | 110 | TCP | POP3 | Post Office Protocol v3 | | 123 | UDP | NTP | Network Time Protocol | | 143 | TCP | IMAP | Internet Message Access Protocol | | 161 | UDP | SNMP | Simple Network Management Protocol | | 162 | UDP | SNMP Trap | SNMP notifications (trap/inform) | | 179 | TCP | BGP | Border Gateway Protocol | | 389 | TCP/UDP | LDAP | Lightweight Directory Access Protocol | | 443 | TCP | HTTPS | HTTP over TLS/SSL | | 445 | TCP | SMB | Server Message Block (file sharing) | | 465 | TCP | SMTPS | SMTP over TLS (implicit) | | 514 | UDP | Syslog | System Logging Protocol | | 587 | TCP | SMTP Submission | Mail submission (STARTTLS) | | 636 | TCP | LDAPS | LDAP over TLS/SSL | | 993 | TCP | IMAPS | IMAP over TLS/SSL | | 995 | TCP | POP3S | POP3 over TLS/SSL | | 1433 | TCP | MS SQL | Microsoft SQL Server | | 1521 | TCP | Oracle DB | Oracle Database listener | | 3306 | TCP | MySQL | MySQL Database | | 3389 | TCP | RDP | Remote Desktop Protocol | | 5060 | TCP/UDP | SIP | Session Initiation Protocol (VoIP) | ### Port Ranges | Range | Name | Description | |-------------------|------------------|---------------------------------------| | 0 - 1023 | Well-Known Ports | Reserved for common services (IANA) | | 1024 - 49151 | Registered Ports | Vendor-specific applications | | 49152 - 65535 | Dynamic/Ephemeral | Temporary client-side ports | --- ## 7. IPv4 Header Diagram ``` 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Version| IHL |Type of Service| Total Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identification |Flags| Fragment Offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Time to Live | Protocol | Header Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options (if IHL > 5) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ``` ### IPv4 Header Fields | Field | Size (bits) | Description | |--------------------|-------------|------------------------------------------------------------| | Version | 4 | IP version (always 4 for IPv4) | | IHL | 4 | Internet Header Length in 32-bit words (min 5 = 20 bytes) | | Type of Service | 8 | DSCP (6 bits) + ECN (2 bits) -- QoS marking | | Total Length | 16 | Total packet size in bytes (header + data), max 65,535 | | Identification | 16 | Fragment identification -- unique per original datagram | | Flags | 3 | Bit 0: Reserved. Bit 1: DF (Don't Fragment). Bit 2: MF (More Fragments) | | Fragment Offset | 13 | Position of fragment in original datagram (in 8-byte units)| | Time to Live (TTL) | 8 | Hop count limit (decremented by 1 at each router) | | Protocol | 8 | Upper-layer protocol (1=ICMP, 6=TCP, 17=UDP, 89=OSPF) | | Header Checksum | 16 | Error detection for header only (recalculated at each hop) | | Source Address | 32 | Sender IP address | | Destination Address | 32 | Receiver IP address | | Options | Variable | Optional (rarely used): record route, timestamp, etc. | ### Common Protocol Numbers | Number | Protocol | Description | |--------|----------|--------------------------------| | 1 | ICMP | Internet Control Message Protocol | | 2 | IGMP | Internet Group Management Protocol | | 6 | TCP | Transmission Control Protocol | | 17 | UDP | User Datagram Protocol | | 47 | GRE | Generic Routing Encapsulation | | 50 | ESP | Encapsulating Security Payload | | 51 | AH | Authentication Header | | 88 | EIGRP | Enhanced Interior Gateway Routing Protocol | | 89 | OSPF | Open Shortest Path First | --- ## 8. Routing Protocol Reference ### Administrative Distance (AD) Table Administrative Distance determines route preference when multiple routing protocols provide a route to the same destination. Lower AD is preferred. | Route Source | Default AD | Notes | |---------------------------------|------------|------------------------------------| | Connected interface | 0 | Directly connected networks | | Static route | 1 | Manually configured routes | | EIGRP summary route | 5 | EIGRP auto-summary | | External BGP (eBGP) | 20 | Routes learned from external AS | | Internal EIGRP | 90 | Routes learned within EIGRP AS | | OSPF | 110 | All OSPF route types | | IS-IS | 115 | Intermediate System to Intermediate System | | RIP | 120 | Routing Information Protocol | | External EIGRP | 170 | Routes redistributed into EIGRP | | Internal BGP (iBGP) | 200 | Routes learned within same AS | | Unknown / Unreachable | 255 | Route will never be used | ### OSPF Quick Reference | Parameter | Value / Details | |------------------------------|------------------------------------------------| | Algorithm | Dijkstra (SPF -- Shortest Path First) | | Metric | Cost (reference bandwidth / interface bandwidth)| | Default reference bandwidth | 100 Mbps | | Protocol number | 89 | | Multicast addresses | 224.0.0.5 (AllSPFRouters), 224.0.0.6 (AllDRouters) | | Hello timer (broadcast) | 10 seconds | | Dead timer (broadcast) | 40 seconds (4x hello) | | Hello timer (NBMA) | 30 seconds | | Dead timer (NBMA) | 120 seconds (4x hello) | | LSA types | Type 1 (Router), Type 2 (Network), Type 3 (Summary), Type 4 (ASBR Summary), Type 5 (External), Type 7 (NSSA External) | | Area 0 | Backbone area (all areas must connect to it) | | DR/BDR election | Highest priority, then highest Router ID | | Default priority | 1 (0 = will not participate in election) | ### OSPF Cost Calculation ``` Cost = Reference Bandwidth / Interface Bandwidth Default Reference Bandwidth = 100 Mbps Examples: 10 Mbps Ethernet: 100 / 10 = 10 100 Mbps FastEthernet: 100 / 100 = 1 1 Gbps GigabitEthernet: 100 / 1000 = 1 (problem! -- increase reference BW) 10 Gbps: 100 / 10000 = 1 (same problem) Recommendation: Set reference bandwidth to 100000 (100 Gbps): router ospf 1 auto-cost reference-bandwidth 100000 ``` ### EIGRP Quick Reference | Parameter | Value / Details | |------------------------------|------------------------------------------------| | Algorithm | DUAL (Diffusing Update Algorithm) | | Metric | Composite: bandwidth, delay, (reliability, load, MTU) | | Protocol number | 88 | | Multicast address | 224.0.0.10 | | Hello timer (high-speed) | 5 seconds | | Hold timer (high-speed) | 15 seconds (3x hello) | | Hello timer (low-speed) | 60 seconds (T1 and below) | | Hold timer (low-speed) | 180 seconds (3x hello) | | Administrative distance | 90 (internal), 170 (external), 5 (summary) | | Maximum hop count | 255 (default 100) | | Load balancing | Equal-cost (default), Unequal-cost (variance) | ### BGP Quick Reference | Parameter | Value / Details | |------------------------------|------------------------------------------------| | Algorithm | Best path selection (multiple attributes) | | Type | Path vector protocol | | Port | TCP 179 | | Administrative distance | 20 (eBGP), 200 (iBGP) | | Neighbour establishment | Manual configuration (no auto-discovery) | | Session types | eBGP (between ASes), iBGP (within AS) | | Keepalive timer | 60 seconds | | Hold timer | 180 seconds | | Path selection order | Weight > Local Preference > Originate > AS Path Length > Origin > MED > eBGP over iBGP > IGP Metric > Router ID | --- ## 9. Spanning Tree Protocol Reference ### STP Port States | Port State | Forwards Data? | Learns MACs? | Sends/Receives BPDUs? | Duration | |---------------|----------------|--------------|----------------------|------------------| | Disabled | No | No | No | Administrative | | Blocking | No | No | Receives only | 20 seconds (Max Age) | | Listening | No | No | Yes | 15 seconds (Fwd Delay) | | Learning | No | Yes | Yes | 15 seconds (Fwd Delay) | | Forwarding | Yes | Yes | Yes | Stable state | **Total convergence time (STP 802.1D):** 20 + 15 + 15 = 50 seconds (worst case) ### STP Timers | Timer | Default Value | Description | |-------------------|---------------|--------------------------------------------------| | Hello Time | 2 seconds | Interval between BPDU transmissions by root | | Max Age | 20 seconds | Maximum age of a BPDU before discard | | Forward Delay | 15 seconds | Time spent in Listening and Learning states each | ### STP Variants | Protocol | Standard | Convergence | Instances | Notes | |---------------|------------|--------------|----------------------|-------------------------------| | STP | 802.1D | 30-50 sec | 1 (CST) | Original, legacy | | PVST+ | Cisco | 30-50 sec | Per VLAN | Cisco proprietary | | RSTP | 802.1w | 1-6 sec | 1 (CST) | Rapid convergence | | Rapid PVST+ | Cisco | 1-6 sec | Per VLAN | Cisco proprietary, rapid | | MSTP | 802.1s | 1-6 sec | Multiple (mapped) | VLANs mapped to instances | ### STP Port Roles | Role | Description | |-----------------|----------------------------------------------------------| | Root Port | Best path to root bridge (one per non-root switch) | | Designated Port | Best port on each segment toward root (forwards traffic) | | Alternate Port | Backup to root port (RSTP -- provides rapid failover) | | Backup Port | Backup to designated port on shared segment (RSTP) | | Blocked Port | Not forwarding (STP 802.1D) | ### Root Bridge Election 1. Lowest Bridge Priority wins (default 32768, configurable in increments of 4096) 2. If priority is tied, lowest MAC address wins 3. Bridge ID = Priority (4 bits) + Extended System ID/VLAN (12 bits) + MAC Address (48 bits) **Setting root bridge:** ``` spanning-tree vlan root primary ! Sets priority to 24576 (or lower) spanning-tree vlan root secondary ! Sets priority to 28672 spanning-tree vlan priority ! Manual priority (increments of 4096) ``` ### PortFast and BPDU Guard | Feature | Command | Purpose | |----------------|---------------------------------------------------|----------------------------------------------| | PortFast | `spanning-tree portfast` | Skip listening/learning, go straight to forwarding | | PortFast (global) | `spanning-tree portfast default` | Enable PortFast on all access ports | | BPDU Guard | `spanning-tree bpduguard enable` | Err-disable port if BPDU is received | | BPDU Guard (global) | `spanning-tree portfast bpduguard default` | Enable BPDU Guard on all PortFast ports | | Root Guard | `spanning-tree guard root` | Prevent port from becoming root port | | Loop Guard | `spanning-tree guard loop` | Prevent alternate/root port from transitioning to designated | **Important:** PortFast should ONLY be enabled on access ports connected to end devices (PCs, printers, servers). Never enable on ports connecting to other switches. --- ## 10. Quick Reference Summary ### Subnetting Shortcuts | If you need... | Then... | |-------------------------|-------------------------------------------------------| | Number of subnets | 2^(borrowed bits) | | Hosts per subnet | 2^(host bits) - 2 | | Subnet mask from CIDR | Set first N bits to 1, rest to 0 | | CIDR from subnet mask | Count the consecutive 1-bits | | Network address | IP AND subnet mask (bitwise AND) | | Broadcast address | Network address OR wildcard mask (bitwise OR) | | Wildcard mask | 255.255.255.255 - subnet mask | | Next subnet | Current network + magic number in interesting octet | ### Binary-Decimal Quick Reference | Binary | Decimal | |------------|---------| | 10000000 | 128 | | 11000000 | 192 | | 11100000 | 224 | | 11110000 | 240 | | 11111000 | 248 | | 11111100 | 252 | | 11111110 | 254 | | 11111111 | 255 | ### Exam Tips 1. **Memorise the powers of 2** from 2^0 to 2^16 at minimum 2. **Memorise the magic numbers:** 128, 64, 32, 16, 8, 4, 2, 1 3. **Always allocate largest subnets first** in VLSM problems 4. **Usable hosts = total - 2** (except /31 and /32) 5. **Network address** always has all host bits set to 0 6. **Broadcast address** always has all host bits set to 1 7. **Wildcard mask** is used in OSPF network statements and ACLs 8. **Practice, practice, practice** -- subnetting speed comes from repetition 9. **Double-check your work** by verifying that the broadcast of one subnet is one less than the network of the next 10. **On the exam:** write out the powers of 2 and magic numbers on your scratch paper immediately --- **Document End** *This document is the intellectual property of VantagePoint Networks. Unauthorised reproduction or distribution is prohibited.* *VantagePoint Networks -- Study Guide Series*