Router
Routers are unique in that they have 2 IP addresses: a public WAN-facing one, and a private LAN-facing one.
- Routers perform the "traffic directing" functions on the Internet, forwarding packets from one network to another
- in other words, data packets are forwarded through the networks of the internet from router to router until they reach their destination computer (with routing algorithms determining the choice of route.)
- A router is like a railroad junction: from one incoming track, there are multiple possible destinations, so which one does it choose? You have to configure that (configurable through port forwarding)
- Each router has a prior knowledge only of networks attached to it directly
- routers gain knowledge of the topology of the network when the routing protocol shares the information of who the router's neighbors are. Each neighbor then shares this information with their neighbors, and so on until the whole network is revealed.
- routing protocols are layer management protocols for L3, regardless of their transport mechanism
- in other words, data may very well travel over L2 or L4
- DSL router - a residential-grade router designed to create LANs and connect them to a WAN (which is provided by the ISP)
- aka residential gateway
- a residential router uses a modem to connect the LAN to the WAN
How routers determine which route to take
- Each packet contains an address in its header, which has a hierarchical structure (like a postal code)
- Each router has a forwarding table which is used to compare against a part of the address to determine the next step in the packet's journey
- Specifically, the routing table is like a hashmap that makes portions of the destination address to outbound links.
- each time a packet arrives at a router, the router consults its routing table.
- This routing table contains the network ID and the host ID
Routing Table
Whenever a node needs to send data to another node on a network, it must first know where to send it. If a direct connection can't be made, then the data has to be sent via other nodes along a route to the destination node.
- Imagine a node somewhere along this chain receives a packet of data. It has no idea where it came from or where it's going. A routing table solves this problem, as it gives each node in the chain the address for the destination node.
- Effectively, the router says "I don't know how to deal with 192.168.0.34, but I know that 192.168.0.254 (a router) knows, so if I get a packet destined for that address, I'll just pass it along to that router, since he knows how to deal with it."
- A routing table is a database that keeps track of paths and uses these to determine which way to forward traffic.
- A routing table is a data file in RAM that is used to store route information about directly connected and remote networks.
Core routers
- core routers are the supercomputers of the internet
- designed to operate on the internet backbone, as opposed to on the edge of a network (edge router, ex. home network).
- the core router's purpose is to forward ip packets along.
- edge routers connect to core routers
Seeing all nodes on a local area network:
We can see all nodes on a local area network with:
sudo nmap -sn 192.168.1.0/24- the 24 is CIDR notation, signifying that we will scan from 192.168.1.0 to 192.168.1.255
- the inclusion of 24 means we are scanning an address block
- the 24 is CIDR notation, signifying that we will scan from 192.168.1.0 to 192.168.1.255
Children
Backlinks