The Archive Ensembl sites, (e.g. http://nov2008.archive.ensembl.org), have the format: http://<three-letter-month><year>.archive.ensembl.org
In the footer of each current Ensembl page, there is a link called 'Permanent link', which links to the corresponding page in the Ensembl Archive. A similar link on each archive page links back to the current site (i.e. www.ensembl.org). Note that in archives released prior to the site redesign in Version 51, these links are in the purple lefthand navigation bar.
Unfortunately, owing to the change in site organisation between releases 50 and 51, it is not always possible to map pages one-to-one between the current Ensembl site and the older archives. If the link does not take you to the data you expected, trying using the search facility to locate the information.
Example:
If you are looking at the Alternative Splicing view for human gene BRCA2 on the
main Ensembl site in September 2008, when Ensembl 51
was the current version, the URL would be:
and the equivalent archived page URL would be:
http://nov2008.archive.ensembl.org/Homo_sapiens/Gene/Splice?db=core;g=ENSG00000139618;r=13:31787617-31871809;t=ENST00000380152