PAT
1. Port Address Translation (PAT)
Overview of PAT
Port Address Translation (PAT), also known as NAT Overload, enables multiple devices within a private network to share a single public IP address when accessing the internet. PAT assigns a unique port number to each connection initiated by an internal device, making simultaneous communication possible through the same public IP.
Differences Between PAT and NAT
| NAT | PAT |
|---|---|
| One-to-one mapping of private to public IPs. | Many-to-one mapping (multiple private IPs to a single public IP using ports). |
| Only the IPv4 address is modified. | Both the IPv4 address and port numbers are modified. |
| Each private device requires a unique public IP. | Multiple devices can share a single public IP. |
Example
In this example, PC1 (192.168.1.10) and PC2 (192.168.1.11) within a private network communicate with an external server. PAT assigns unique source ports (4545 for PC1 and 5656 for PC2) to differentiate their sessions.
2. Configuring PAT
PAT with Single Public IP
In this scenario, devices on the internal network (192.168.1.0/24) share a single public IP (203.20.10.103) for internet access. PAT distinguishes each session with unique port numbers.
Configuration Steps:
- Create an Access List for Internal Traffic:
Router(config)# access-list 1 permit 192.168.1.0 0.0.0.255
2. Enable PAT (Overload) on the Outside Interface:
Router(config)# ip nat inside source list 1 interface GigabitEthernet0/1 overload
3. Identify Inside and Outside Interfaces:
Router(config)# interface GigabitEthernet0/0 Router(config-if)# ip nat inside Router(config)# interface GigabitEthernet0/1 Router(config-if)# ip nat outside
Verifying the Configuration:
Router# show ip nat translations Pro Inside global Inside local Outside local Outside global --- 203.20.10.103 192.168.1.10:1025 201.20.10.1:80 201.20.10.1:80 --- 203.20.10.103 192.168.1.11:1026 201.20.10.1:80 201.20.10.1:80
View NAT Statistics
Router# show ip nat statistics Total active translations: 2 (0 static, 2 dynamic, 0 extended) Outside interfaces: GigabitEthernet0/1 Inside interfaces: GigabitEthernet0/0 Hits: 20 Misses: 1 Expired translations: 0 Dynamic mappings: Inside Source access-list 1 interface GigabitEthernet0/1 overload
PAT with Multiple Public IPs
While commonly used with a single public IP, PAT can be configured with a pool of public IP addresses. This setup provides scalability in networks with heavy outbound traffic.
Configuration Steps:
- Define Inside and Outside Interfaces:
Router(config)# interface GigabitEthernet0/1 Router(config-if)# ip nat inside Router(config)# interface GigabitEthernet0/0 Router(config-if)# ip nat outside
2. Create a Pool of Public IP Addresses:
Router(config)# ip nat pool PUBLIC_POOL 203.20.10.103 203.20.10.105 netmask 255.255.255.0
3. Define the Access List:
Router(config)# access-list 1 permit 192.168.1.0 0.0.0.255
4. Configure PAT Using the Public IP Pool:
Router(config)# ip nat inside source list 1 pool PUBLIC_POOL overload
Verifying PAT Configuration
After configuring PAT, it’s essential to verify its functionality and troubleshoot any issues.
View Active NAT Translations
Router# show ip nat translations Pro Inside global Inside local Outside local Outside global --- 203.20.10.103:4545 192.168.1.10:4545 201.20.10.1:80 201.20.10.1:80 --- 203.20.10.104:5656 192.168.1.11:5656 201.20.10.1:443 201.20.10.1:443 --- 203.20.10.105:7692 192.168.1.12:7692 201.20.10.1:80 201.20.10.1:80 --- 203.20.10.103:4803 192.168.1.13:4803 201.20.10.2:80 201.20.10.2:80 --- 203.20.10.104:9873 192.168.1.14:9873 201.20.10.3:443 201.20.10.3:443
This displays the current NAT translations, showing which internal IPs are using which public IPs and ports.
View NAT Statistics
Router# show ip nat statistics
Total active translations: 5 (0 static, 5 dynamic, 0 extended)
Outside interfaces: GigabitEthernet0/0
Inside interfaces: GigabitEthernet0/1
Hits: 50 Misses: 3
Expired translations: 0
Dynamic mappings:
Inside Source access-list 1 pool PUBLIC_POOL overload
Pool PUBLIC_POOL: netmask 255.255.255.0
Start: 203.20.10.103 End: 203.20.10.105
Type: natpool, total addresses: 3, allocated: 2 (1% in use)
Explanation of Output:
- Total active translations: Shows the current number of active NAT translations.
- Outside interfaces / Inside interfaces: Specifies the interfaces configured for NAT.
- Hits: Successful NAT translation requests.
- Misses: Failed translation attempts.
- Dynamic mappings: Details of the dynamic NAT pool, including the IP range and allocation usage.