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3.1.2. Route Selection and Address Translation
When multiple routes exist to the same destination, routers select based on this hierarchy:
- Longest prefix match (most specific): /26 beats /24, regardless of anything else
- Administrative distance (AD): Lower = more trusted (used when prefixes are equal)
- Metric: Lower = better (used when AD is equal)
Administrative Distance Values:
| Source | AD | Trust Level |
|---|---|---|
| Connected | 0 | Directly attached—absolute trust |
| Static | 1 | Administrator configured—high trust |
| EIGRP | 90 | Cisco dynamic routing |
| OSPF | 110 | Standard dynamic routing |
| RIP | 120 | Legacy protocol—low trust |
⚠️ Exam Trap: Prefix length always wins. A /26 route is used over a /24 route even if the /24 has lower AD. The exam tests whether you understand this hierarchy—don't be fooled by attractive-looking AD values when prefix lengths differ.
Address Translation:
| Type | Function | Use Case |
|---|---|---|
| NAT | Translates one IP to another (1:1) | Map internal server to public IP |
| PAT | Maps many internal IPs to one external IP using ports | Home routers, office internet access |
PAT (Port Address Translation), also called NAT overload, is what most home and office routers use. Internal devices share one public IP, differentiated by source port numbers.
Written byAlvin Varughese
Founder•15 professional certifications