Chapter 4 · Watch, then practise
Network Layer
Work from IPv4 addresses and prefix lengths to forwarding decisions. Then examine how a datagram is divided when its size exceeds a link’s MTU.
3 questions · 3 with related videos. Matches are based on playlist titles; broader background matches are labeled.
What to study
- CIDR and subnet masks
- Longest-prefix forwarding
- IPv4 fragmentation
Chapter playlists
Choose a playlist
Notes
Lec-50: Variable Length Subnet Masking(VLSM) in Hindi with Examples | Computer Networks
Gate Smashers · 14:03
Subnetting examples support the /26 calculation; the address in this question is worked separately.
1. Find an IPv4 subnet
For 192.168.10.77/26, find the mask, network, broadcast and conventional usable host range.
A /26 mask is 255.255.255.192, leaving 6 host bits and blocks of 64 addresses. The address lies in 192.168.10.64/26. Broadcast is .127; conventional host addresses are .65–.126, giving 62 hosts. These host-count conventions do not apply unchanged to /31 point-to-point links.
4.2 What's inside a router? Part 1.
JimKurose · 14:37 · Background lecture
Supplementary background on router input processing and destination-based forwarding. Apply the longest-prefix rule to the original routing table below; this is not a solution to that specific example.
2. Longest matching prefix
A router has routes for 10.0.0.0/8 and 10.2.0.0/16. Which matches destination 10.2.3.4 most specifically?
Both match, but /16 fixes more leading address bits, so 10.2.0.0/16 is selected by longest-prefix matching. A default /0 route is used only if no more specific matching route is available.
Lec-55: Fragmentation(MTU) of IPv4 Datagram | Identification, Flags and Fragment Offset | Networks
Gate Smashers · 13:22
Supplementary lecture specifically covers IPv4 fragmentation, MTU, flags and offsets; use the datagram sizes supplied here.
3. Fragmentation calculation
An IPv4 datagram is 4,000 bytes including a 20-byte header. MTU is 1,500, options are absent and fragmentation is allowed. Find payload sizes and offsets.
Payload is 3,980 bytes. Each nonfinal fragment can carry 1,480 bytes, divisible by eight. Payloads are 1,480, 1,480 and 1,020 bytes; total lengths are 1,500, 1,500 and 1,040. Offsets are 0, 185 and 370 in eight-byte units. MF flags are 1, 1, 0.