Question 1

What is an autonomous system number (ASN)?

Accepted Answer

A globally unique identifier for a **network under a single routing policy** — an ISP, a large hosting company, a university, a bank. It is the unit in which the internet's routing table is expressed, and it is how one network refers to another.

**What an autonomous system is.** A collection of IP address ranges administered by one organisation that presents a **consistent routing policy** to the outside world. Internally it may be enormously complex; externally it appears as a single entity making one set of decisions about which traffic it will carry.

**What the number does.** BGP — the protocol networks use to tell each other which addresses they can reach — describes every route as an **AS path**: the sequence of autonomous systems a packet would traverse. A route advertisement effectively says "to reach these addresses, send traffic through AS 1, then AS 2, then AS 3."

**This makes the AS path the internet's map.** It is how routers choose between paths, how loops are prevented — a network rejects any route that already contains its own number — and how operators diagnose where traffic is actually going.

**Who gets one.** Any organisation that connects to **more than one provider** and wants to control its own routing, or that operates address space independently. You apply to your **regional internet registry**, justify the need, and pay a membership fee.

**Numbers were originally 16-bit**, giving roughly 65,000, which ran out; **32-bit ASNs** are now standard.

**Why it matters practically:**

**Multi-homing.** Having your own ASN and address space is what lets you change provider without renumbering, and stay up when one provider fails.

**Diagnostics.** Looking up which AS an address belongs to tells you whose network you are actually dealing with.

**Security.** Route hijacks are described in terms of the wrong AS announcing someone else's prefixes, and **RPKI** binds prefixes to the ASNs authorised to announce them.

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

Who actually runs the internet?

Accepted Answer

Nobody, and several bodies with narrow remits — which is the accurate answer rather than an evasive one. There is no central authority; there are institutions that coordinate specific scarce

Question 4

What is anycast, and how can one IP address be in many places?

Accepted Answer

Anycast announces the same IP address from multiple locations, and the routing system naturally delivers each user's traffic to whichever location is closest in network terms. It is one of the more

Question 5

How does a phone number actually get routed?

Accepted Answer

Through a hierarchy of lookups that once corresponded directly to geography and physical switches, and now mostly corresponds to database entries — a change that number portability forced. The

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 a MAC address, and how is it different from an IP address?

Accepted Answer

A MAC address identifies a piece of hardware on a local network; an IP address identifies a location on the internet. One is fixed to the device, the other describes where the device currently is —

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