Created Address Space (markdown)

howto/Address-Space.md

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+DN42 uses network addresses in the [rfc1918](https://tools.ietf.org/html/rfc1918) and [ULA](https://tools.ietf.org/html/rfc4193) ranges. These are described in detail in the sections below. 
+
+The [DN42 registry](https://git.dn42.us/dn42/registry) is the authoritative source of information on address space assignment. Within the registry, the DN42 address space is divided in to blocks based on _policies_ that define how the addresses may be used. Policies are defined in `inetnum` and `inet6num` objects and can be:
+
+ - **open** - users may request prefixes in this range, subject to any constraints that are described in the `remark` attributes
+ - **closed** - these ranges cannot be assigned
+ - **reserved** - these ranges are reserved for future use
+ - **ask** - these ranges are for specific uses, please ask on the mailing list before requesting assignments
+
+The [filter.txt](https://git.dn42.us/dn42/registry/src/master/data/filter.txt) and [filter6.txt](https://git.dn42.us/dn42/registry/src/master/data/filter6.txt) files within the registry detail the network wide constraints on what address ranges are in use together with the global limits on what can be announced. 
+
+`inetnum` and `inet6num` objects within the registry are used to describe the allocation of address space to users. `route` and `route6` objects in the registry are used to validate routing announcements through [ROA](https://wiki.dn42/howto/Bird#route-origin-authorization). 
+
+In addition to the native DN42 address ranges, the registry also contains allocations for the address space used by affiliate networks. These are updated by a regular [sync script](https://git.dn42.us/dn42/registry-sync). 
+
+Globally routable prefixes are not supported in DN42; they are denied via the registry filter{6,}.txt files and many networks will filter both announcements and traffic for prefixes that are outside of the allowable ranges.
+
+# IPv6 Address Space
+
+DN42 uses the fd00::/8 ULA range for IPv6 addresses; the whole fd00::/8 block has an open policy and users are free to request any prefix in this range, that is not already allocated. 
+
+**The DN42 registry is not authoritative for the fd00::/8 range**
+
+DN42 is interconnected with other networks, like icvpn, which also use the same ULA range and many users will also use this range for their own networks. A registration in the dn42 registry cannot prevent IPv6 conflicts, so a fully random prefix (see [RFC4193](https://tools.ietf.org/html/rfc4193)) is strongly recommended. If an address conflict is found, then needing to renumber your network is no fun.
+
+# IPv4 Address Space
+
+DN42 uses the 172.20.0.0/14 range for IPv4 addresses. As with the public internet, IPv4 space is limited and users are encouraged to conserve space where possible. 
+
+Unlike the IPv6 address space, the DN42 IPv4 space is not fully open for assignment to users; some ranges are intended for specific uses and other ranges are reserved. See the policy section, below. Users should always check the policy in the registry before requesting a prefix to be assigned. 
+
+There are other IPv4 ranges in use within DN42 related to the affiliate networks, see the [filter.txt](https://git.dn42.us/dn42/registry/src/master/data/filter.txt) file in the registry. 
+
+## IPv4 Policies
+
+The diagram below shows the allocation policies for the DN42 address space. 
+
+... image ...
+
+Specific policy restrictions:
+
+| Prefix | Usage |
+|--------|-------|
+| 172.20.0.0/24<br/>172.21.0.0/24<br/>172.22.0.0/24<br/>172.23.0.0/24 | Reserved for anycast addresses |
+| 172.20.240.0/20<br/>172.22.240.0/20 | Reserved for transfer networks |
+| 172.20.64.0/18 | Reserved for allocations larger than /23, up to /21 |
+| 172.22.0.0/18 | Reserved for allocations of /24 or larger, up to /21 |
+| 172.23.16.0/21 | Closed to new allocations |
+
+