FAQ ON NAT & PAT ROUTING# INTERVIEW PREP GUIDE ABOUT BGP ROUTING
🔹 What is BGP ?
BGP (Border Gateway Protocol) is a path-vector routing protocol used to exchange routing information between Autonomous Systems (AS).
🔹 What is NAT ?
NAT (Network Address Translation) translates private IP addresses to public IP addresses.
🔹 What is PAT ?
PAT (Port Address Translation) maps multiple private IPs to a single public IP using ports.
🔹 Why is NAT used ?
To conserve public IPv4 addresses and enable internet access for private networks.
🔹 Why is PAT called NAT overload ?
Because many internal hosts share one public IP via different port numbers.
🔹 Where is NAT configured ?
On edge routers or firewalls connecting to the internet.
🔹 What problem does NAT solve ?
IPv4 address exhaustion.
🔹 What is a private IP address ?
An address used inside local networks, not routable on the internet.
🔹 What is a public IP address ?
An address routable over the internet.
🔹 Does NAT modify packet headers ?
Yes, it changes source or destination IP (and ports in PAT).
🔹 Is NAT required in IPv6 ?
Generally no, due to large address space.
🔹 What is Static NAT ?
One-to-one mapping between private and public IP.
🔹 What is Dynamic NAT ?
Maps private IPs to a pool of public IPs.
🔹 What is PAT (NAT Overload) ?
Many-to-one mapping using ports.
🔹 What is Twice NAT ?
Both source and destination addresses are translated.
🔹 What is Policy NAT ?
NAT applied based on defined policies (ACLs, routes, etc.).
🔹 What is Destination NAT (DNAT) ?
Changes the destination IP in incoming traffic.
🔹 What is Source NAT (SNAT) ?
Changes the source IP in outgoing traffic.
🔹 What is Port Forwarding ?
Forwarding specific ports from public IP to internal server.
🔹 What is NAT Pool ?
A range of public IPs used for dynamic NAT.
🔹 What is Identity NAT ?
Traffic passes without translation.
🔹 How does PAT differentiate sessions ?
By assigning unique source port numbers.
🔹 What is a NAT table ?
A table maintaining mappings of private-to-public addresses and ports.
🔹 What happens when the NAT table is full ?
New sessions cannot be translated.
🔹 Does NAT affect performance ?
Yes, due to translation overhead.
🔹 What protocols may break with NAT ?
Protocols embedding IP info (e.g., FTP, SIP).
🔹 What is NAT traversal ?
Techniques to allow traffic through NAT devices.
🔹 What is a timeout in NAT ?
The duration a mapping stays active without traffic.
🔹 Does NAT provide security ?
Indirectly hides internal IPs but is not a firewall.
🔹 Can NAT work with UDP ?
Yes.
🔹 Can NAT work with TCP ?
Yes.
🔹 When should Static NAT be used ?
For hosting internal servers accessible from the internet.
🔹 When should Dynamic NAT be used ?
When multiple public IPs are available.
🔹 When should PAT be used ?
When only one public IP is available.
🔹 What is hairpin NAT ?
Internal users accessing internal server via public IP.
🔹 What is NAT loopback ?
Same as hairpin NAT.
🔹 Can two internal hosts use the same port in PAT ?
Yes, PAT assigns different external ports.
🔹 How is NAT configured in routers ?
Using inside and outside interfaces and rules.
🔹 How does NAT work with DHCP clients ?
Clients receive private IPs and are translated by NAT.
🔹 Does NAT affect VPN ?
Yes, may require NAT-T (NAT Traversal).
🔹 What is NAT exemption ?
Traffic that bypasses NAT rules.
📚 Advanced Concepts
What is Carrier-Grade NAT (CGNAT) ?
NAT performed by ISPs to share public IPs among customers.
🔹 What is symmetric NAT ?
Each connection gets a unique mapping.
🔹 What is full-cone NAT ?
One mapping allows any external host to respond.
🔹 What is restricted NAT ?
Only specific external hosts can reply.
🔹 What is port-restricted NAT ?
Replies allowed only from specific IP and port.
🔹 How does NAT help IPv4 exhaustion ?
By allowing reuse of private IPs.
🔹 What logs are important in NAT troubleshooting ?
NAT translation table and session logs.
🔹 What commands verify NAT ?
Commands showing NAT table and statistics.
🔹 What is the main limitation of NAT ?
Breaks end-to-end connectivity principle.
🔹 Is NAT still widely used today ?
Yes, in almost all IPv4 networks.
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