Routing

Network layer path selection — how packets find their way

Hop-by-Hop Path traceroute

Source 192.168.1.10 Router Hop 1 192.168.1.1 Router Hop 2 10.0.0.1 Router Hop 3 72.14.215.1 Destination 142.250.80.46 
$ traceroute example.com
 1  192.168.1.1     1.234 ms
 2  10.0.0.1        8.567 ms
 3  72.14.215.1    12.891 ms
 4  142.250.80.46  15.234 ms

Routing Table netstat -rn

Destination Gateway Interface Metric 0.0.0.0/0 192.168.1.1 eth0 100 192.168.1.0/24 0.0.0.0 eth0 0 10.0.0.0/8 192.168.1.254 eth0 50
FieldDescription
DestinationNetwork address with CIDR prefix
GatewayNext hop IP (0.0.0.0 = directly connected)
InterfaceNetwork interface to send packets through
MetricCost/priority (lower = preferred)

Default Gateway 0.0.0.0/0

Local Network 192.168.1.0/24 Host A .10 Host B .20 Gateway .1 Internet 
Packet to 8.8.8.8:
  1. Check routing table
  2. No specific match found
  3. Use default route 0.0.0.0/0
  4. Forward to gateway 192.168.1.1

CIDR Notation RFC 4632

192.168.1.0/24 Network Portion (24 bits) Host (8 bits) 11000000.10101000.00000001. xxxxxxxx Network: Fixed Host: Variable
CIDRSubnet MaskAddressesUsable
/8255.0.0.016,777,21616,777,214
/16255.255.0.065,53665,534
/24255.255.255.0256254
/32255.255.255.25511

Private IP Ranges RFC 1918

10.0.0.0/8 10.0.0.0 - 10.255.255.255 Class A 172.16.0.0/12 172.16.0.0 - 172.31.255.255 Class B 192.168.0.0/16 192.168.0.0 - 192.168.255.255 Class C
RangeNetworksTotal AddressesCommon Use
10.0.0.0/8116,777,216Large enterprises
172.16.0.0/12161,048,576Medium networks
192.168.0.0/1625665,536Home/small office

Network Address Translation RFC 3022

Private 192.168.1.10 :54321 NAT Translation Table Public 203.0.113.5 :12345
DirectionPrivate AddressPublic Address
Outbound192.168.1.10:54321203.0.113.5:12345
Inbound192.168.1.10:54321203.0.113.5:12345

BGP Basics RFC 4271

AS 64501 ISP Alpha AS 64502 Transit AS 64503 ISP Beta eBGP eBGP AS Path: 64501 64502 64503
AttributeDescriptionSelection
AS PathList of AS numbers traversedShorter = preferred
Local PrefPreference within ASHigher = preferred
MEDMulti-Exit DiscriminatorLower = preferred
OriginIGP, EGP, or IncompleteIGP > EGP > Incomplete
BGP Path Selection (simplified):
  1. Highest Local Preference
  2. Shortest AS Path
  3. Lowest Origin type
  4. Lowest MED
  5. eBGP over iBGP
  6. Lowest router ID