What are private IP addresses?

Private IP addresses are IP addresses intended for use inside local networks that are not routed directly to the Internet.

Short and clear: private IP addresses are used inside a network and are not visible on the Internet.

---

#Why private IP addresses are needed

Private IP addresses allow you to:

• build local networks (at home, in an office, in a data center)
• use the same IP addresses in different networks
• save public IPv4 addresses
• isolate an internal network from the Internet

---

#Ranges of private IP addresses (IPv4)

The standard (RFC 1918) defines three ranges:

#1. 10.0.0.0 - 10.255.255.255

10.0.0.0/8

• a very large range
• often used in large networks

---

#2. 172.16.0.0 - 172.31.255.255

172.16.0.0/12

• a medium-sized range

---

#3. 192.168.0.0 - 192.168.255.255

192.168.0.0/16

• the most popular
• used in home and office networks

---

#What is important to understand

• private IP addresses:
  • are not globally unique
  • can repeat in different networks
• routers on the Internet:
  • do not accept packets with private IPs
  • such packets are dropped

---

#How private IPs reach the Internet

NAT (Network Address Translation) is used to access the Internet:

• inside the network: a private IP
• outside: a public IP
• the router substitutes the addresses

That is exactly why:

• dozens of devices can access the Internet
• using a single public IP address

---

#Private addresses and IPv6

Important for an interview

• IPv6 also has local addresses
• but the concept is different
• NAT in IPv6 is not mandatory

---

#In short, for an interview

Private IP addresses are ranges of IP addresses intended for use inside local networks that are not routed to the Internet and usually reach outside through NAT.