To prevent conflicts in new namespaces added by extensions, please list your extension's custom namespaces here. If you define a constant for a custom namespace, list it here too.
Note that these are only default namespaces. All extensions defining new namespaces should provide the installer with a method of configuring the extension to use a different range of namespaces for its custom namespaces.
Naturally some namespaces listed on this page will overlap, and violate the recommendations listed here. The recommendations are good practices new extensions should try to adhere to to avoid conflicts; for legacy reasons, old extensions that have already created conflicts and bad practice can't simply be changed but are listed here to help avoid even more conflicts.
Note: Namespaces using numbers from 3000 and higher are meant to be used by system administrators to define their custom namespaces. Thus extension developers should not use this range.
The namespaces in 100-199 are reserved for site-specific namespaces, and should not be used by extensions. Many extensions are already using this range, though, so be careful. If you're writing a new extension you should never hardcode any namespaces inside the 100-199 range.
Many Wikimedia Foundation wikis have site-specific namespaces in this range. Including the Portal namespace (100) on most Wikipedia projects, Creator namespace (100) on Wikimedia Commons, the Extension namespace (102) on mediawiki.org, the Subject namespace (112) on English Wikibooks, and many others.
300 - 399. Wikia has been seen using namespaces in this range for its custom extensions and configurations. To avoid conflicts in case Wikia extensions are ported to work outside of Wikia it would be best to avoid defining new extensions that use this range. Wikia alters the namespaces for some extensions used on its own wikis. Please also take these into account while coming up with new namespace numbers to use in new extensions.
Additional JSON data. As of 2015-02[update] you can store arbitrary valid JSON in the subnamespace Data:Json, e.g. Data:Json:MyDataset. This is a developing convention on Wikimedia projects that use JsonConfig and Graph. Note: Constant name also used by: LinkedWiki
The 1000+ range starts to get fairly high and is a range some sites may attempt to use to avoid conflicts with extensions. So be wary of defining new namespaces within this range, especially near 1000.
Note:The BlueSpiceExtensions bundle contains only two extensions with a namespace at the moment. This may change in the near future. Therefore the Hallo Welt! team would appreciate it if the range of 1500-1599 could be reserved for this package.
So far, no known extensions have defined default namespaces in the 3000+ range. For now, it would be best to avoid using this range for extensions, so that sites can use this range to define their custom namespaces without fear of conflict.