Networking Essentials

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:

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:

1. Green/White (TX+)
2. Green (TX-)
3. Orange/White (RX+)
4. Blue
5. Blue/White
6. Orange (RX-)
7. Brown/White
8. Brown

3. Apply OSI Model

Seven Layers (Top to Bottom):

Remember: "All People Seem To Need Data Processing"

4. Configure TCP/IP Addressing

IP Address Classes:

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

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