Network Architecture and Security

Internet structure

The Internet can be described according to the following structure:

Network edge: formed by hosts and servers
Access networks: wired and wireless communication links provide access to the Internet
Network core: a mesh of interconnected routers which switch packets and interconnect access networks and network edge

Internet uses Packet-switching to send data:

Hosts break application-layer messages into packets
Forward packets from one router to the next, across links on path from source to destination

At center (network core): small number of well-connected large networks

“tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage
Content provider network (e.g., Google): private network that connects its data centers to Internet, often bypassing tier-1, regional ISPs
Internet Exchange Points (IXP) which interconnect Tiers
Points of Presence (PoP) where Tiers’ clients interconnect

The figure shows how the network core is formed by a set of connected large networks at different levels. At a higher level the tier 1 networks interconnects tier 2 networks. A big internet service provider can be a tier 2 network which itself provides access to tier 3 networks and Internet end-users. Tier 3 networks are multi-homed internet service providers which give access to end-users. The figure also shows points of presence and internet exchange points. The points of presence are the physical locations in tier 2 networks where users interconnect. The internet exchange points interconnect tier networks.

Figure 1 Structure of the Internet Core Network
Click to enlarge
View on tier 1 and 2 ISP interconnections by Ludovic.ferre from Wikipedia CC BY-SA 3.0

Users connect to Internet through access networks: Public Switched Telephone network, Cable Operator, ADSL …

This figure presents different access networks which connect to tier 3 networks.

Figure 2 Different Access Networks connect to Internet Service Providers
Click to enlarge
Diagram showing how customers connect to ISP's... by Ludovic.ferre from Wikipedia CC BY-SA 3.0