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+.. _cgnat:
+
+#####
+CGNAT
+#####
+
+:abbr:`CGNAT (Carrier-Grade Network Address Translation)` , also known as
+Large-Scale NAT (LSN), is a type of network address translation used by
+Internet Service Providers (ISPs) to enable multiple private IP addresses to
+share a single public IP address. This technique helps to conserve the limited
+IPv4 address space.
+The 100.64.0.0/10 address block is reserved for use in carrier-grade NAT
+
+Overview
+========
+
+CGNAT works by placing a NAT device within the ISP's network. This device
+translates private IP addresses from customer networks to a limited pool of
+public IP addresses assigned to the ISP. This allows many customers to share a
+smaller number of public IP addresses.
+
+Not all :rfc:`6888` requirements are implemented in CGNAT.
+
+Implemented the following :rfc:`6888`  requirements:
+
+- REQ 2: A CGN must have a default "IP address pooling" behavior of "Paired".
+  CGN must use the same external IP address mapping for all sessions associated
+  with the same internal IP address, be they TCP, UDP, ICMP, something else,
+  or a mix of different protocols.
+- REQ 3: The CGN function should not have any limitations on the size or the
+  contiguity of the external address pool.
+- REQ 4: A CGN must support limiting the number of external ports (or, 
+  equivalently, "identifiers" for ICMP) that are assigned per subscriber
+
+Advantages of CGNAT
+-------------------
+
+- **IPv4 Address Conservation**: CGNAT helps mitigate the exhaustion of IPv4 addresses by allowing multiple customers to share a single public IP address.
+- **Scalability**: ISPs can support more customers without needing a proportional increase in public IP addresses.
+- **Cost-Effective**: Reduces the cost associated with acquiring additional public IPv4 addresses.
+
+Considerations
+--------------
+
+- **Traceability Issues**: Since multiple users share the same public IP address, tracking individual users for security and legal purposes can be challenging.
+- **Performance Overheads**: The translation process can introduce latency and potential performance bottlenecks, especially under high load.
+- **Application Compatibility**: Some applications and protocols may not work well with CGNAT due to their reliance on unique public IP addresses.
+- **Port Allocation Limits**: Each public IP address has a limited number of ports, which can be exhausted, affecting the ability to establish new connections.
+- **Port Control Protocol**: PCP is not implemented.
+
+Port calculation
+================
+
+When implementing CGNAT, ensuring that there are enough ports allocated per subscriber is critical. Below is a summary based on RFC 6888.
+
+1. **Total Ports Available**:
+
+   - Total Ports: 65536 (0 to 65535)
+   - Reserved Ports: Assume 1024 ports are reserved for well-known services and administrative purposes.
+   - Usable Ports: 65536 - 1024 = 64512
+
+2. **Estimate Ports Needed per Subscriber**:
+
+   - Example: A household might need 1000 ports to ensure smooth operation for multiple devices and applications.
+
+3. **Calculate the Number of Subscribers per Public IP**:
+
+   - Usable Ports / Ports per Subscriber
+   - 64512 / 1000 ≈ 64 subscribers per public IP
+
+
+Configuration
+=============
+
+.. cfgcmd:: set nat cgnat pool external <pool-name> external-port-range <port-range>
+
+    Set an external port-range for the external pool, the default range is 
+    1024-65535. Multiple entries can be added to the same pool.
+
+.. cfgcmd:: set nat cgnat pool external <pool-name> external-port-range per-user-limit port <num>
+
+    Set external source port limits that will be allocated to each subscriber
+    individually. The default value is 2000.
+
+.. cfgcmd:: set nat cgnat pool external <pool-name> range [address | address range | network]
+
+    Set the range of external IP addresses for the CGNAT pool.
+
+.. cfgcmd:: set nat cgnat pool internal <pool-name> range [address range | network]
+
+    Set the range of internal IP addresses for the CGNAT pool.
+
+.. cfgcmd:: set nat cgnat pool rule <num> source pool <internal-pool-name>
+
+    Set the rule for the source pool.
+
+.. cfgcmd:: set nat cgnat pool rule <num> translation pool <external-pool-name>
+
+    Set the rule for the translation pool.
+
+
+
+Configuration Examples
+======================
+
+Single external address
+-----------------------
+
+Example of setting up a basic CGNAT configuration:
+In the following example, we define an external pool named `ext-1` with one external IP address
+
+
+Each subscriber will be allocated a maximum of 2000 ports from the external pool.
+
+.. code-block:: none
+
+   set nat cgnat pool external ext1 external-port-range '1024-65535'
+   set nat cgnat pool external ext1 per-user-limit port '2000'
+   set nat cgnat pool external ext1 range '192.0.2.222/32'
+   set nat cgnat pool internal int1 range '100.64.0.0/28'
+   set nat cgnat rule 10 source pool 'int1'
+   set nat cgnat rule 10 translation pool 'ext1'
+
+Multiple external addresses
+---------------------------
+
+.. code-block:: none
+
+   set nat cgnat pool external ext1 external-port-range '1024-65535'
+   set nat cgnat pool external ext1 per-user-limit port '8000'
+   set nat cgnat pool external ext1 range '192.0.2.1-192.0.2.2'
+   set nat cgnat pool external ext1 range '203.0.113.253-203.0.113.254'
+   set nat cgnat pool internal int1 range '100.64.0.1-100.64.0.32'
+   set nat cgnat rule 10 source pool 'int1'
+   set nat cgnat rule 10 translation pool 'ext1'
+
+
+
+Further Reading
+===============
+
+- :rfc:`6598` - IANA-Reserved IPv4 Prefix for Shared Address Space
+- :rfc:`6888` - Requirements for CGNAT
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