Assuming you're using one of the "standard" bibliography styles -- e.g.,
alpha, or a style that's derived from one of these styles -- you shouldn't be using the
url field at all: the style in use doesn't recognize
url as a valid field type and thus won't do anything with its contents. If you do want to use the
url field, be sure to use one of many newer style files, such as those provided by the
Still assuming you're using one of BibTeX's "standard" styles mentioned above, do note that the field
howpublished is only acted on if the entry type is
@misc; nothing happens with this field if the entry is of type
@article, etc. If you're using one of the latter entry types, you should use the
note field to store the URL-related information.
I strongly recommend you use a newer style file that does know what to do with fields of type
url. You can then use the
note field for supplementary information, such as when a given website was last accessed.