Router

• It has IP and MAC address on each NIC
• Since it has to connect to two different networks it must have different IPs simultaneously, hence multiple NICs are required
• Internet is nothing but a bunch of routers

Routing Table

• Routing Table is stored in router or network host
• When Routers receive packets with an unknown Destination IP, packet is dropped
• Hence, Routing Table must be populated ahead of time unlike ARP Cache
• https://en.wikipedia.org/wiki/Routing_table
• MacOS routing table

netstat -rn
Routing tables
 
Internet:
Destination        Gateway            Flags               Netif Expire
default            10.119.41.223      UGScg               utun4       
default            192.168.31.1       UGScIg                en0       
default            link#25            UCSIg           bridge100      !
10.119.11.1        10.119.41.223      UGHS                utun4       
10.119.41.223/32   127.0.0.1          UGSc                  lo0       
13.107.64/18       192.168.31.1       UGSc                  en0       
52.112/14          192.168.31.1       UGSc                  en0       
52.122/15          192.168.31.1       UGSc                  en0       
127                127.0.0.1          UCS                   lo0       
127.0.0.1          127.0.0.1          UH                    lo0       
165.85.126.240     192.168.31.1       UGHS                  en0       
192.168.31         10.119.41.223      UGdCSc              utun4   

• Windows routing table

route print

• netstat tips tricks
  • https://serverfault.com/questions/11289/netstat-tips-and-tricks

Populating Routing Table

• It can be populated by 3 methods
  • Directly Connected
    • Routes for the Networks which are attached
  • Static routes
    • Routes manually provided by an Administrator
  • Dynamic routes
    • Routes learned automatically from other Routers
    • Exact method to learn is governed by different protocols called Dynamic Routing Protocols

	Destination Network ID	Next Hop
DC	10.0.55.x /24	Left
DC	10.0.44.x /24	Right
Dyn. or Static	10.0.66.x /24 (This network is not directly available)	10.0.55.2 (Another router’s IP address)

Default Gateway

• A default gateway is each host’s way out of their local network
• If all the forwarding rules are not applicable to the destination address, then default gateway is chosen as the last resort
• Generally default gateway is a router, since routers connect to the other networks and the internet
• Routing decision on two default gateways: https://serverfault.com/questions/59442/routing-decision-when-there-are-two-default-gateways-on-windows

Getting IP address of WiFi router

• Look for default gateway
• MacOS - List routing table entries
  • https://stackoverflow.com/questions/6782658/how-to-get-default-gateway-in-mac-osx
  • netstat -rn
• Windows
  • ipconfig /all

Getting strength of Wifi

• MacOS: Option+Click on Wifi Symbol
• Calculated using RSSI (Received Signal Strength Indicator)
• measured in decibels (dBm)
• Max strength: -30dBm
• Very good strength: -67 dBm
• Okay: -70 dBm
• https://www.metageek.com/training/resources/understanding-rssi/

Router Hierarchy

• Routers are typically connected in a hierarchy
• Helps network easier to scale
• Helps to have more consistent connectivity
• Hierarchy allows for Route Summarization
  • Reduce the number of Routes in Routing Table
  • Default Route - Ultimate route summary
    • 0.0.0.0/0 — for everything else go here

R5 Routing Table

Destination Network ID	Next Hop
10.40.55.0/24	R6
10.40.66.0/24	R7
10.40.77.0/24	R8
10.20.55.0/24	R4
10.20.66.0/24	R4
10.20.77.0/24	R4

• After Route Summarization

Destination Network ID	Next Hop
10.40.55.0/24	R6
10.40.66.0/24	R7
10.40.77.0/24	R8
10.20.0.0/16	R4

R8 Routing Table

Destination Network ID	Next Hop
10.40.77.0/24	DC
10.40.55.0/24	R5
10.40.66.0/24	R5
10.20.55.0/24	R5
10.20.66.0/24	R5
10.20.77.0/24	R5

• After Route Summarization

Destination Network ID	Next Hop
10.40.77.0/24	DC
10.0.0.0/8	R5

• Using Default Route

Destination Network ID	Next Hop
10.40.77.0/24	DC
0.0.0.0/0	R5