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:

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):

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