Networking Essentials

Purpose

Provide foundational networking knowledge required for penetration testing including OSI model concepts, TCP/IP addressing, subnetting, cable standards, and Cisco device configuration. Enable understanding of network infrastructure for effective security assessments.

Inputs/Prerequisites

Basic computer literacy
Access to network devices or simulator (Packet Tracer, GNS3)
Understanding of binary and hexadecimal notation
Network topology information for target environment

Outputs/Deliverables

Properly configured network infrastructure
Subnetting calculations and IP schemes
VLAN segmentation plans
Router and switch configurations
Network documentation for assessments

Core Workflow

1. Understand Network Types

Network Categories:

LAN (Local Area Network): Single location, high speed
WAN (Wide Area Network): Multiple locations, geographic dispersion
Internet: Global interconnection of networks

LAN Architectures:

Client/Server: Centralized resources and management
Peer-to-Peer: Decentralized, each device shares resources

2. Master Cable Standards

UTP Cable Categories:

| Category | Speed | Use Case | |----------|-------|----------| | CAT 3 | 10 Mbps | Ethernet | | CAT 5 | 100 Mbps | Fast Ethernet | | CAT 5e | 1 Gbps | Gigabit Ethernet | | CAT 6 | 1 Gbps | Multi-Gigabit |

Cable Wiring Standards:

Straight-Through (T568A to T568A):

Use: PC to Switch, Router to Switch
Pin 1-8 same on both ends

Crossover (T568A to T568B):

Use: PC to PC, Switch to Switch, Router to Router
Transmit/Receive pairs swapped

T568A Pinout:

Green/White (TX+)
Green (TX-)
Orange/White (RX+)
Blue
Blue/White
Orange (RX-)
Brown/White
Brown

3. Apply OSI Model

Seven Layers (Top to Bottom):

| Layer | Name | Function | Protocols/Devices | |-------|------|----------|-------------------| | 7 | Application | User interface | HTTP, FTP, SMTP | | 6 | Presentation | Data formatting | SSL, JPEG, ASCII | | 5 | Session | Connection management | NetBIOS, RPC | | 4 | Transport | End-to-end delivery | TCP, UDP | | 3 | Network | Routing and addressing | IP, ICMP, Routers | | 2 | Data Link | Frame transmission | Ethernet, Switches | | 1 | Physical | Bit transmission | Cables, Hubs |

Remember: "All People Seem To Need Data Processing"

4. Configure TCP/IP Addressing

IP Address Classes:

| Class | Range | Default Mask | Networks | |-------|-------|--------------|----------| | A | 1-126 | 255.0.0.0 | Large organizations | | B | 128-191 | 255.255.0.0 | Medium organizations | | C | 192-223 | 255.255.255.0 | Small organizations |

Private IP Ranges:

Class A: 10.0.0.0 - 10.255.255.255
Class B: 172.16.0.0 - 172.31.255.255
Class C: 192.168.0.0 - 192.168.255.255

Subnetting Calculation:

Network: 192.168.1.0/26
Subnet Mask: 255.255.255.192

Subnets: 2^2 = 4 subnets
Hosts per subnet: 2^6 - 2 = 62 hosts

Subnet 1: 192.168.1.0 - 192.168.1.63
Subnet 2: 192.168.1.64 - 192.168.1.127
Subnet 3: 192.168.1.128 - 192.168.1.191
Subnet 4: 192.168.1.192 - 192.168.1.255

5. Configure Cisco Routers

Basic Router Commands:

! Enter privileged mode
Router> enable
Router#

! Enter configuration mode
Router# configure terminal
Router(config)#

! Set hostname
Router(config)# hostname R1

! Configure interface
Router(config)# interface FastEthernet 0/0
Router(config-if)# ip address 192.168.1.1 255.255.255.0
Router(config-if)# no shutdown

! Save configuration
Router# copy running-config startup-config

! Show commands
Router# show ip interface brief
Router# show running-config
Router# show ip route

Configure Static Routing:

Router(config)# ip route 10.0.0.0 255.0.0.0 192.168.1.2
Router(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.1  ! Default route

Configure RIP Routing:

Router(config)# router rip
Router(config-router)# version 2
Router(config-router)# network 192.168.1.0
Router(config-router)# network 10.0.0.0

6. Configure Cisco Switches

Basic Switch Commands:

! Configure management VLAN
Switch(config)# interface vlan 1
Switch(config-if)# ip address 192.168.1.10 255.255.255.0
Switch(config-if)# no shutdown

! Configure port
Switch(config)# interface FastEthernet 0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10

VLAN Configuration:

! Create VLAN
Switch(config)# vlan 10
Switch(config-vlan)# name SALES
Switch(config-vlan)# exit

! Assign port to VLAN
Switch(config)# interface range fa0/1-10
Switch(config-if-range)# switchport access vlan 10

! Configure trunk port
Switch(config)# interface GigabitEthernet 0/1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20,30

Inter-VLAN Routing (Router-on-a-Stick):

Router(config)# interface FastEthernet 0/0.10
Router(config-subif)# encapsulation dot1Q 10
Router(config-subif)# ip address 192.168.10.1 255.255.255.0

Router(config)# interface FastEthernet 0/0.20
Router(config-subif)# encapsulation dot1Q 20
Router(config-subif)# ip address 192.168.20.1 255.255.255.0

7. Configure Access Lists

Standard ACL (Filter by Source IP):

! Deny specific host
Router(config)# access-list 10 deny host 192.168.1.100
Router(config)# access-list 10 permit any

! Apply to interface
Router(config)# interface FastEthernet 0/0
Router(config-if)# ip access-group 10 in

Extended ACL (Filter by Source, Destination, Port):

! Block Telnet from specific network
Router(config)# access-list 100 deny tcp 192.168.1.0 0.0.0.255 any eq 23
Router(config)# access-list 100 permit ip any any

! Apply to interface
Router(config-if)# ip access-group 100 in

8. Configure NAT

Static NAT:

Router(config)# ip nat inside source static 192.168.1.10 203.0.113.10
Router(config)# interface FastEthernet 0/0
Router(config-if)# ip nat inside
Router(config)# interface Serial 0/0
Router(config-if)# ip nat outside

Dynamic NAT with PAT:

Router(config)# access-list 1 permit 192.168.1.0 0.0.0.255
Router(config)# ip nat pool MYPOOL 203.0.113.1 203.0.113.1 netmask 255.255.255.0
Router(config)# ip nat inside source list 1 pool MYPOOL overload

Quick Reference

Common Subnet Masks

| CIDR | Mask | Hosts | |------|------|-------| | /24 | 255.255.255.0 | 254 | | /25 | 255.255.255.128 | 126 | | /26 | 255.255.255.192 | 62 | | /27 | 255.255.255.224 | 30 | | /28 | 255.255.255.240 | 14 | | /29 | 255.255.255.248 | 6 | | /30 | 255.255.255.252 | 2 |

Essential Show Commands

show ip interface brief
show running-config
show ip route
show vlan brief
show interfaces trunk
show access-lists
show ip nat translations

Constraints

VLAN hopping attacks possible without proper trunk security
Weak ACLs can be bypassed
NAT can complicate penetration testing
Routing protocols can be exploited if not secured
Switch CAM tables can be flooded

Examples

Example 1: Basic Network Scan Understanding

# Scan reveals network structure
nmap -sn 192.168.1.0/24

# Identify VLANs through ARP
arp-scan -l

# Discover routing
traceroute 10.0.0.1

Example 2: VLAN Enumeration

# Check for VLAN hopping vulnerability
yersinia -G

# DTP attack (if trunk misconfigured)
yersinia dtp -attack 1

Troubleshooting

| Issue | Solution | |-------|----------| | No connectivity | Verify cable type (straight vs crossover) | | Cannot reach gateway | Check IP configuration and subnet mask | | VLAN isolation | Verify trunk configuration and allowed VLANs | | Routing not working | Check ip routing enabled and routes configured | | ACL blocking traffic | Review access-list and interface application | | NAT not translating | Verify inside/outside interface designation |