Linux Networking Fundamentals

Network Hardware Fundamentals

Switches: These devices enable communication among multiple hosts within a network. All hosts connected to a single switch form a local area network (LAN).

For communication to occur:

Each host must have a MAC address (physical address) conssiting of 12 hexadecimal characters (0-9, A-F)
Communication via switches may sometimes require broadcasting
Excessive broadcasting can impact host performance
Note: Communication always involves both sending and receiving data

Routers: These devices facilitate communication between hosts in different local networks.

For communication to occur:

Each host must have an IP address (logical address) represented in decimal format (e.g., 192.168.10.1)
IP addresses consist of network information (LAN identifier) + host address information
Communication relies on routing tables within the router

Router Table Generation

Routing table information can be generated through several methods:

Automatic generation from directly connected networks
Manual configuration (static routing)
Dynamic generation through routing protocols (dynamic routing)

Gateway: The essential path for a host to access other networks. The router's interface serves as the gateway intefrace, and the router's address is the gateway address.

Static Routing Configuration

Network Environment Setup

Consider a network with:

Two hosts
Two switches
Three routers

Router Configuration

Initial router setup involves configuring interface addresses:


Router> enable          # Enter privileged mode
Router# configure terminal  # Enter global configuration mode
Router(config)# interface gigabitethernet 0/0  # Enter interface configuration mode
Router(config-if)# ip address 192.168.1.254 255.255.255.0  # Configure IP address
Router(config-if)# no shutdown  # Enable interface
Router(config-if)# exit  # Return to global configuration mode

Router(config)# interface gigabitethernet 0/1
Router(config-if)# ip address 192.168.2.1 255.255.255.0
Router(config-if)# no shutdown

To verify interface status:


Router# show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol 
GigabitEthernet0/0     192.168.1.254   YES manual up                    up 
GigabitEthernet0/1     192.168.2.1     YES manual up                    down

Static Route Configuration

To configure a static route, use the following syntax:


ip route destination_network subnet_mask next_hop_address

Example configuration for Router 1:


Router(config)# ip route 192.168.4.0 255.255.255.0 192.168.2.2

To verify the routing table:


Router# show ip route
S    192.168.4.0/24 [1/0] via 192.168.2.2

Removing Static Routes

To remove a static route:


Router(config)# no ip route 192.168.4.0 255.255.255.0 192.168.2.2

Dynamic Routing Configuration

Dynamic routing protocols enable routers to share routing information automatically. Common protocols include:

RIP (Routing Information Protocol)
OSPF (Open Shortest Path First)
EIGRP (Enhanced Interior Gateway Routing Protocol)
BGP (Border Gateway Protocol)

RIP Configuration Example

For Router 1:


Router(config)# router rip
Router(config-router)# network 192.168.1.0
Router(config-router)# network 192.168.2.0

To view learned routes:


Router# show ip route
R    192.168.3.0/24 [120/1] via 192.168.2.2, 00:00:09, GigabitEthernet0/1
R    192.168.4.0/24 [120/2] via 192.168.2.2, 00:00:20, GigabitEthernet0/1

Network Topology Hierarchy

Networks are typically designed in three hierarchical layers:

Core Layer: Comprises routers (gateway interfaces) that handle external communication with built-in redundancy capabilities
Distribution Layer: Contains layer 3 switches that provide redundancy and policy control
Access Layer: Consists of layer 2 switches that connect end devices to the network

Network Models

OSI 7-Layer Model

From top to bottom:

Application Layer: Defines application interfaces and standards (data generation)
Presentation Layer: Handles data conversion, encryption, and compression
Session Layer: Manages network connection establishment and termination
Transport Layer: Ensures reliable data transmission
Network Layer: Provides routing capabilities to find target networks (layer 3 devices)
Data Link Layer: Provides switching capabilities to find target hosts (layer 2 devices)
Physical Layer: Defines physical device standards (network cards, cables, fiber optics) and converts data to binary

TCP/IP Model (4-Layer)

Application Layer
Host-to-Host Layer
Internet Layer
Network Access Layer

TCP vs. UDP Protocols

TCP (Transmission Control Protocol): Connection-oriented protocol

Establishes connection before data transfer (e.g., online file sending)
Advantage: High data transmission reliability
Disadvantage: Lower data transmission efficiency

UDP (User Datagram Protocol): Connectionless protocol

Sends data without establishing a connection (e.g., offline file sending)
Advantage: High data transmission efficiency
Disadvantage: Lower data transmission reliability

TCP Connection Process

Three-way handshake:

Host A sends a TCP packet with SYN flag set to 1 to request connection
Host B responds with a TCP packet having both SYN and ACK flags set to 1
Host A sends a TCP packet with ACK flag set to 1, confirming receipt of Host B's response

Four-way termination:

First FIN: One host sends a FIN packet to request connection termination
First ACK: The other host acknowledges the FIN request
Second FIN: The other host sends its own FIN packet to terminate the connection
Second ACK: The first host acknowledges the second FIN

System Routing Configuration

CentOS 6 (route command)

Default static route:


route add default gw 10.0.0.2
route del default gw 10.0.0.2

Network static route:


route add -net 10.0.3.0 netmask 255.255.255.0 gw 10.0.1.2
route del -net 10.0.3.0 netmask 255.255.255.0 gw 10.0.1.2

Host static route:


route add -host 10.0.3.201 dev eth1
route del -host 10.0.3.201 dev eth1

CentOS 7 (ip route command)