Network Routing Static, OSPF, BGP, NAT

What is Routing?

Routing is the process of selecting the best path for data packets to travel from source to destination across one or more networks. Routers use routing tables to make forwarding decisions based on destination IP addresses.

Static vs Dynamic Routing

Feature	Static Routing	Dynamic Routing
Configuration	Manually configured by admin	Automatically learned via routing protocols
Adaptability	Does not adapt to network changes	Adapts automatically to topology changes
Overhead	No routing protocol overhead	Routing protocol traffic overhead
Scalability	Poor (manual updates needed)	Excellent
Security	More secure (no protocol to exploit)	Less secure (routing updates can be spoofed)
Use Case	Small networks, stub networks, default routes	Large, complex networks

Dynamic Routing Protocols

IGP = Interior Gateway Protocol (within an autonomous system) | EGP = Exterior Gateway Protocol (between autonomous systems)

Protocol	Type	Algorithm	Use Case
RIP	IGP, Distance Vector	Bellman-Ford	Small networks (max 15 hops)
OSPF	IGP, Link State	Dijkstra (SPF)	Large enterprise networks
EIGRP	IGP, Hybrid	DUAL	Cisco networks
BGP	EGP, Path Vector	Best Path Selection	Internet backbone, ISPs
IS-IS	IGP, Link State	Dijkstra	Large ISP networks

Distance Vector vs Link State

Feature	Distance Vector	Link State
Knowledge	Only knows neighbors and distances	Complete topology map
Updates	Periodic full table updates to neighbors	Triggered updates (LSAs) flooded to all
Convergence	Slow	Fast
Memory/CPU	Low	High
Loops	Prone to routing loops	Loop-free (SPF algorithm)
Examples	RIP, IGRP	OSPF, IS-IS

NAT - Network Address Translation

NAT allows multiple devices on a private network to share a single public IP address. It translates private IP addresses to public IP addresses and vice versa.

SNAT (Source NAT): Translates the source IP address. Used when private hosts access the Internet. The router replaces the private source IP with its public IP.
DNAT (Destination NAT): Translates the destination IP address. Used for port forwarding - incoming traffic to a public IP is redirected to a private server.
PAT (Port Address Translation) / NAT Overload: Maps multiple private IPs to a single public IP using different port numbers. Most common form of NAT in home routers.

Default Gateway

The default gateway is the router that a device uses to send traffic to destinations outside its local network. When a device doesn't have a specific route for a destination, it sends the packet to the default gateway.

Example: If your PC has IP 192.168.1.100/24 and default gateway 192.168.1.1, all traffic to non-192.168.1.x addresses goes to 192.168.1.1 (your router).

Apply Longest-Prefix Match

from ipaddress import ip_address, ip_network

routes = [
    (ip_network('0.0.0.0/0'), 'wan'),
    (ip_network('10.0.0.0/8'), 'corp'),
    (ip_network('10.24.0.0/16'), 'engineering'),
]
destination = ip_address('10.24.8.9')
matching = [(network, hop) for network, hop in routes if destination in network]
network, hop = max(matching, key=lambda route: route[0].prefixlen)
print(network, hop)

Output

10.24.0.0/16 engineering

Diagnose a Missing Return Route

Forward path: branch 10.30.0.0/16 -> core -> service 172.20.4.10
Observed: request reaches the service, but no reply reaches the branch.
Check: the service gateway has no route for 10.30.0.0/16.
Correction: advertise or add the return route, then verify both directions without relying on NAT as a guess.

A successful outbound trace does not establish that the reverse path is routable.

Before you move on