Ref: https://learn.cantrill.io/courses/2022818/lectures/45699909

• Summary:
• Contents:

IPv4 Address Space

• IPv4 standardized in 1981
  • Standardized via RFC 791 document: https://datatracker.ietf.org/doc/html/rfc791
  • IPv4 still most popular L3 protocol nowadays… but big shortage of IPv4 addresses
• IPv4 Address Range = 0.0.0.0→255.255.255.255 = ~4.3 billion addresses
  • 💡 ~1 IPv4 address per 2 people in the world… Far from enough!
  • Originally fully-managed by IANA (Internet Assigned Numbers Authority)…
    • …nowadays parts delegated to regional authorities (RIPE, ARIN…)
• ‼️ Public IPv4 addresses are allocated by agencies!
  • If you want to use a Public IP, you need an agency to allocate it for you
  • Private IPv4 addresses, in contrast, can be used & reused freely in private NWs

Historical Segregation of IPv4 Address Space

💡 No need to memorize these, more of historical background

• Class A NWs (0.X.X.X, 1.X.X.X, …, 127.X.X.X) → Prefix = /8
  • More than 16 million IPs per NW → Huge NWs
  • Historically allocated to large business with early internet presence (e.g. Apple, Ford Motor, US military…)
    • Nowadays many organizations gave up these NWs, and IPs in this range delegated to regional managers
• Class B NWs (128.0.X.X, 128.1.X.X, …, 191.254.X.X, 191.255.X.X) → Prefix = /16
  • More than 65k IPs per NW
  • Historically used by large businesses without need for Class A NW (nowadays many of these IPs also allocated by regional authorities)
• Class C NWs (192.0.1.X, 192.0.2.X, …, 223.255.254.X, 223.255.255.X) → Prefix = /24
  • 256 IPs per NW
  • Historically used by smaller business with need for internet presence
• Class D NWs → Used for multicast
• Class E Nws → Reserved

Private IPv4 Address Ranges

• 🔧 Private IPv4 addresses can NOT be routed across public internet! (reserved for use in private NWs)
  • Ranges defined in RFC 1918 document: https://datatracker.ietf.org/doc/html/rfc1918
  • For devices in private NWs (which have private IPv4 addresses) to communicate with public internet, Network Address Translation (NAT) is needed
• ‼️ Memorize these ranges!! You must be able to see an IPv4 address and recognize it as a public or private IPv4 address!!
  1. 10.0.0.0-10.255.255.255 = 10.X.X.X = 10.0.0.0/8
     • 💡 It's a single class A NW
     • Often used in cloud environments
  2. 172.16.0.0-172.31.255.255 = 172.16.0.0/12
     • 💡 Total of 16 class B NWs
     • ❗ Default VPC in AWS uses 172.31.X.X = 172.31.0.0/16, memorize that!
  3. 192.168.0.0-192.168.255.255 = 192.168.X.X = 192.168.0.0/16
     • 💡 Total of 256 class C NWs
     • Generally used within home and small office NWs
       • 💡 Typical range for old-school LAN parties! :)