Question 1

What is MTU, and why does the wrong setting break some websites?

Accepted Answer

**Maximum transmission unit** is the largest packet size a link will carry — and a mismatch produces one of the most confusing faults in networking: most things work, but some websites hang after loading part of the page.

**The standard sizes.** Ethernet's MTU is **1500 bytes**. Some connections carry less: PPPoE, used on many broadband services, adds 8 bytes of overhead and reduces the usable MTU to **1492**; VPN tunnels subtract more; some mobile and satellite links are lower again.

**What happens to an oversized packet.** Traditionally, a router could **fragment** it into pieces and the destination would reassemble them. In IPv4 this is still possible unless the **Don't Fragment (DF)** flag is set — and in practice it usually is, because fragmentation is inefficient and problematic. **IPv6 does not permit routers to fragment at all.**

So instead, the router **discards** the packet and sends back an ICMP **"Fragmentation Needed"** message stating the maximum size it can carry. The sender then reduces its packet size for that destination. This mechanism is **Path MTU Discovery**.

**Why it fails.** The mechanism depends entirely on that ICMP message getting back. Many firewalls **block all ICMP** as a blunt security measure. When that happens:

**The oversized packets vanish silently.**

**The sender never learns why**, and keeps retransmitting at the same size.

**Small packets get through fine.** The handshake succeeds, the connection establishes, and small responses arrive.

**Large transfers hang.** The page starts loading and then stops.

This is the **PMTUD black hole**, and its signature is unmistakable: some sites work perfectly, others connect and then stall, and the problem follows the network rather than the device.

**Where it shows up most:** VPN connections, PPPoE broadband, tunnelled connections, and some hosting configurations.

**How to diagnose it.** Send progressively larger pings with fragmentation disabled and find where they stop being answered. The largest that succeeds, plus header overhead, is your usable path MTU.

**Fixes:** lower the MTU on the router or VPN interface; enable **MSS clamping**, which rewrites TCP's advertised segment size so it fits — the standard remedy on consumer routers; and stop blocking all ICMP.

Question 2

What is BGP, and why does it sometimes break the internet?

Accepted Answer

Border Gateway Protocol is how independent networks tell each other which addresses they can reach. It is the routing system the entire internet depends on — and it works largely on trust, which is

Question 3

What is actually inside a network packet?

Accepted Answer

Layers of headers wrapped around your data, each added by a different part of the networking stack and each read by a different piece of equipment along the way. Why layers exist. Each layer solves

Question 4

Why did the internet run out of IPv4 addresses, and how are they traded now?

Accepted Answer

Because the address space was fixed at 32 bits — about 4.3 billion addresses — in 1981, when the internet was a research network and nobody expected every household to hold dozens of connected

Question 5

What is traceroute actually telling you?

Accepted Answer

The path your packets take toward a destination, hop by hop — and it is genuinely useful for diagnosis, provided you know which parts of it to ignore. How it works — a clever trick. Every IP packet

Question 6

What are peering and transit?

Accepted Answer

The two ways networks connect to each other — and the distinction is essentially who pays whom. The internet is a network of networks. No single organisation runs it. Around a hundred thousand

Question 7

What is Quality of Service on a router?

Accepted Answer

A set of techniques for deciding which traffic gets priority when there is not enough capacity for everything — because the only time prioritisation matters is when a link is full. The problem. A

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