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    IPv4 Addressing, Subnetting, NAT and Fragmentation Short Notes for GATE CS

    IPv4 Addressing, Subnetting, NAT and Fragmentation short notes for GATE CS: 3 study cards covering concepts, formulas, shortcuts and exam traps, plus solved p

    ipv4 addressing subnetting nat and fragmentation short notes

    Summary: IPv4 Fragmentation Quick Reference

    Quick Reference

    Original Data
    Total Length IP Header (usually 20 bytes).
    Max Data per Fragment
    MTU IP Header.
    Adjusted Max Data ()
    Largest multiple of 8 Max Data per Fragment.
    Number of Fragments
    \lceil \text{Original Data} / D_{max} \rceil
    Fragment Offset
    (Starting byte of data in original datagram) .
    MF Flag
    1 for all fragments except the last one (which is 0).
    DF Flag
    1 means do not fragment; drop and send ICMP error if too large.
    Multi-router Rule
    Each fragment is evaluated independently against the new MTU. All sub-fragments share the original Identification number.

    Summary: Header Modifications and NAT

    Quick Reference Summary

    Header Modifications at a Router

    • TTL: Always decremented by 1. (Packet dropped if it reaches 0).
    • Header Checksum: Always recalculated if any header field changes.
    • Source/Dest IP: Unchanged by standard routers. Modified by NAT routers.
    • Protocol: Never changed in transit.

    The Protocol Field

    1 → ICMP 6 → TCP 17 → UDP

    Network Address Translation (NAT)

    • Purpose: Conserves public IPv4 addresses by allowing many private hosts to share one public IP.
    • Private Ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16.
    • NAPT / PAT: Translates both IP and Port numbers. Uses a translation table to map internal (IP, Port) to external (IP, Port).
    • Side Effect: NAT modifies the IP (and Port), which means the TCP/UDP checksum must also be recalculated by the NAT router, as the pseudo-header has changed.

    Summary: CIDR and Subnetting Cheat Sheet

    Final Revision Checklist

    Total Addresses
    Usable Hosts
    Prefix Length
    Valid Start IP Multiple of
    Valid Mask Contiguous 1s then 0s
    Routing Tie-breaker Longest Prefix
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    IPv4 Addressing, Subnetting, NAT and Fragmentation Short Notes for GATE CS

    IPv4 Addressing, Subnetting, NAT and Fragmentation short notes for GATE CS: 3 study cards covering concepts, formulas, shortcuts and exam traps, plus solved practice questions.

    Summary: IPv4 Fragmentation Quick Reference

    Quick Reference

    Original Data
    Total Length IP Header (usually 20 bytes).
    Max Data per Fragment
    MTU IP Header.
    Adjusted Max Data ()
    Largest multiple of 8 Max Data per Fragment.
    Number of Fragments
    \lceil \text{Original Data} / D_{max} \rceil
    Fragment Offset
    (Starting byte of data in original datagram) .
    MF Flag
    1 for all fragments except the last one (which is 0).
    DF Flag
    1 means do not fragment; drop and send ICMP error if too large.
    Multi-router Rule
    Each fragment is evaluated independently against the new MTU. All sub-fragments share the original Identification number.

    Summary: Header Modifications and NAT

    Quick Reference Summary

    Header Modifications at a Router

    • TTL: Always decremented by 1. (Packet dropped if it reaches 0).
    • Header Checksum: Always recalculated if any header field changes.
    • Source/Dest IP: Unchanged by standard routers. Modified by NAT routers.
    • Protocol: Never changed in transit.

    The Protocol Field

    1 → ICMP 6 → TCP 17 → UDP

    Network Address Translation (NAT)

    • Purpose: Conserves public IPv4 addresses by allowing many private hosts to share one public IP.
    • Private Ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16.
    • NAPT / PAT: Translates both IP and Port numbers. Uses a translation table to map internal (IP, Port) to external (IP, Port).
    • Side Effect: NAT modifies the IP (and Port), which means the TCP/UDP checksum must also be recalculated by the NAT router, as the pseudo-header has changed.

    Summary: CIDR and Subnetting Cheat Sheet

    Final Revision Checklist

    Total Addresses
    Usable Hosts
    Prefix Length
    Valid Start IP Multiple of
    Valid Mask Contiguous 1s then 0s
    Routing Tie-breaker Longest Prefix

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