1.7 Given a scenario, use appropriate IPv4 network addressing
Address Types
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Public vs. Private IPv4 Addresses
When dealing with IP addresses, computers and devices need a way to communicate over networks. IPv4 addresses are 32-bit numbers that uniquely identify devices. These addresses are divided into Public and Private types, depending on where and how they are used.
1. Public IPv4 Addresses
Definition:
A Public IPv4 address is an IP address that is unique on the internet. These addresses are routable across the public internet, meaning devices with these addresses can communicate directly with other devices anywhere in the world.
Key Points:
- Assigned by Internet Service Providers (ISPs).
- Must be globally unique to avoid conflicts.
- Used for devices that need to be directly reachable from the internet, such as:
- Web servers
- Email servers
- Network devices that host services accessed externally
Example of a Public IP:
172.217.10.46 (one of Google’s public IP addresses)
Exam Tip:
You may be asked which IP type can be accessed from the internet. The answer is Public IP.
2. Private IPv4 Addresses
Definition:
A Private IPv4 address is an IP address used within a private network. These addresses cannot be routed on the internet, so they are only visible and usable inside the internal network.
Key Points:
- Commonly used in home networks, office networks, and enterprise LANs.
- Helps conserve public IP addresses because multiple networks can reuse the same private IP ranges.
- Devices with private IPs access the internet through Network Address Translation (NAT) on a router or firewall.
Private IPv4 Address Ranges:
These are defined by the RFC 1918 standard:
| IP Class | Private Range | Number of Addresses |
|---|---|---|
| A | 10.0.0.0 – 10.255.255.255 | 16,777,216 |
| B | 172.16.0.0 – 172.31.255.255 | 1,048,576 |
| C | 192.168.0.0 – 192.168.255.255 | 65,536 |
Example of a Private IP:
192.168.1.25 (commonly used in home Wi-Fi networks)
Exam Tip:
You may be asked which IP type cannot be routed on the internet. The answer is Private IP.
3. How Public and Private Addresses Work Together
In real IT networks:
- Devices inside a private network are given private IP addresses.
- When they need to access the internet, a router or firewall uses NAT (Network Address Translation) to map the private IP to a public IP.
- The response from the internet is then translated back to the private IP so the device receives it.
Example Scenario:
- Internal device IP:
192.168.1.100 - Router public IP:
203.0.113.5 - Device accesses
www.example.com→ router translates the private IP to its public IP → traffic goes to the internet → response comes back → router sends it to192.168.1.100.
Key Concept for Exam:
- Private IPs cannot be used on the internet directly.
- Public IPs are required for external internet communication.
- NAT bridges private and public IP communication.
4. Summary Table (Exam-Friendly)
| Feature | Public IP | Private IP |
|---|---|---|
| Scope | Internet-wide | Internal networks only |
| Assigned by | ISP | Network administrator / DHCP |
| Can be routed on Internet | Yes | No |
| Example | 203.0.113.5 | 192.168.1.100 |
| Common Use | Web servers, external services | PCs, printers, internal servers |
| RFC Standard | N/A | RFC 1918 |
5. Exam Tips
- Remember private ranges: 10.x.x.x, 172.16–31.x.x, 192.168.x.x.
- Know that private addresses need NAT to communicate with public networks.
- Public IPs are limited and managed by ISPs; private IPs are reusable in multiple networks.
- If a question asks about “IP that can be used inside a company but not on the internet,” the answer is Private IP.
