AS numbers: update wording

draft-dekater-scion-controlplane.md

@@ -321,29 +321,31 @@ A suitable mechanism to globally coordinate the assignation of ISD numbers does
 Currently, ISD numbers are allocated by Anapaya, the Swiss-based provider of SCION-based networking software and solutions.
 
 
-### AS Numbers
+### SCION AS Numbers
 
-An AS number is the 48-bit identifier for an AS. Although they play a similar role, there is no relationship between SCION AS numbers and BGP ASNs as defined by RFC4893. For historical reasons some autonomous systems use a SCION AS number and a BGP ASN that are numerically identical. There is no technical requirement for such an equality.
+A SCION AS number is the 48-bit identifier for an AS. Although they play a similar role, there is no relationship between SCION AS numbers and BGP ASNs as defined by RFC4893. For historical reasons some SCION autonomous systems use a SCION AS number where the first 16 bits are 0, and the remaining 32 bits are identical to their BGP ASN. There is no technical requirement for such an equality.
 
 
-#### Formatting
+#### Text Representation
 
-The default formatting for AS numbers in SCION is very similar to IPv6 (see {{RFC5952}}). It uses a 16-bit colon-separated lower-case hex encoding with leading 0's omitted: `0:0:0` to `ffff:ffff:ffff`.
+The default text representation for SCION AS numbers is very similar to IPv6 (see {{RFC5952}}). It uses a 16-bit colon-separated lower-case hex encoding with leading 0's omitted: `0:0:0` to `ffff:ffff:ffff`.
 
 In SCION, the following rules apply:
 
 - The `::` zero-compression feature of IPv6 is NOT allowed. The feature has very limited use in a 48-bit address space and would only add more complexity.
-- For historical reasons, SCION AS numbers in the lower 32 bit range may be represented as decimal for human consumption. For example, if a program receives the AS number `0:1:f`, it may display the number as "65551".
 - A range of AS numbers can be shortened with a notation similar to the one used for CIDR IP ranges ({{RFC4632}}). For example, the range of the lowest 32-bit AS numbers (0-4294967295) can be represented as `0:0:0/16`.
 
-The next table gives an overview of the AS number allocation.
+For historical reasons, SCION AS numbers in the lower 32 bit range may also be represented as decimal for human readability. For example, if a program receives the AS number `0:1:f`, it may display the number as "65551".
+
+
+#### Special-Purpose SCION AS Numbers
 
 | AS               | Size        | Description                                                                 |
 |------------------+-------------+-----------------------------------------------------------------------------|
-| 0                | 1           | The wildcard AS                                                             |
-| 1-4294967295     | ~4.3 bill.  | 32-bit SCION AS numbers                                                |
+| `0:0:0`          | 1           | The wildcard AS                                                             |
+| `0:0:1-0:ffff:ffff`| ~4.3 bill.  | Public SCION AS numbers                                              |
 | `1:0:0`          | 1           | Reserved                                                                    |
-| `2:0:0/16`       | ~4.3 bill.  | Additional 32-bit range of SCION AS numbers                            |
+| `2:0:0/16`       | ~4.3 bill.  | Additional public SCION AS numbers                                     |
 | `ff00:0:0/32`    | 65535       | Reserved for documentation and test/sample code (analogous to {{RFC5398}}). |
 | `ff00:0:0/24`    | ~16.8 mill. | Reserved for private use (analogous to {{RFC6996}}). These numbers can be used for testing/private deployments. |
 | `ffff:ffff:ffff` | 1           | Reserved                                                                    |