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There seems to be a growing interest in SEO for subdomains. I find that ironic given that, after many years of tolerating subdomain abuse, Google has over the past 12 months or so tightened its requirements for subdomain visibility and inclusion. I would say a lot of people have missed the boat, but there are certainly still legitimate uses for subdomains and, when their content becomes large enough, there are legitimate uses for sitemaps on subdomains.

So let’s talk about sitemaps for subdomains. We’ll start with traditional HTML sitemaps.

Use HTML sitemaps on any subdomain with more than 10 pages – You can include an HTML sitemap on a smaller site but it doesn’t provide much value. However, once you get up to the 10-15 page volume for a site, your on-page navigation becomes clumsy and inefficient for your visitors. Yes, search engines can still crawl 30-50 links on a page and they are happy to do it, but you need to keep your users in mind. They tend to be overwhelmed by more than 10-12 navigational options.

An HTML sitemap can act like an extension of your primary site navigation. You link to the most important content in your sitewide navigation, and your HTML sitemap should be included in your idea of your most important content. Make it easy for people to find the HTML sitemap. These are among the few types of pages where people are mentally comfortable with navigating through dozens or hundreds of links.

Speaking of hundreds of links on a page…

Paginate large HTML Sitemaps – Just because you CAN put 300 links on a page doesn’t mean you should. It’s okay to paginate your HTML sitemaps when they become very large. Google recommends keeping your links to around 100 per page. That’s as good a limit as 150, 200, or 300 but it’s more user-friendly. Think of ways to organize your paginated HTML sitemap so that people can quickly and easily find what they are looking for.

If your site is extremely large, with hundreds of thousands or millions of pages, you might consider creating a subdomain that is used as nothing other than an HTML sitemap. Think of how you could structure and populate the pages so that people want to use them. You might use a date-oriented structure to archive aging content, a topic-oriented structure, an alphabetized structure, or a hybrid structure.

Include subdomains in your primary HTML sitemap – If your main site is large and has its own HTML sitemap, it’s okay to include at least the top-level content from your subdomains in your sitemap page(s). If the subdomains are integrated into your architecture and design, it makes sense to include them in your primary HTML sitemap. You don’t need to include everything on a subdomain in that highest level sitemap. You can deep-link to your subdomain’s HTML sitemap pages and its root URL and leave it at that.

XML Sitemaps for Subdomains – If you have enough content on your subdomain to justify an HTML sitemap, you have enough content to justify an XML sitemap. You can link to your subdomain pages from your primary subdomain XML sitemaps but you may find you have to authenticate each subdomain with the various search engines (Google, Live, and Yahoo! all allow you to do this).

Subdomain sitemaps can be hosted on the subdomain or primary domain, but again you may run into search engine restrictions. Be sure you authenticate your sites properly and try to manage your subdomain XML sitemaps on the subdomains themselves.

Paginate XML Sitemaps – There is divided opinion in the SEO community on whether you should include as many URLs as possible in XML sitemaps or if you should break them up. Think about this in terms of what is easiest for you to manage. On a large content site you’re most likely going to have sections by topic, date, or some other categorical structure. I would recommend dividing your URLs into XML sitemaps that match your major site structures.

You can do this on large content subdomains as well. The rule of thumb is, ask yourself which is more unwieldy: scrolling through 50,000 blocks of XML code or updating 1,000 XML files.

Only repeat your most important pages in multiple XML sitemaps – An XML sitemap helps a search engine identify URLs for crawling. If you have more than one XML sitemap on a site, I see no reason not to include the root URL and HTML sitemap in every XML sitemap. I do this myself. I feel strongly that covering those URLs as much as possible is better than assuming I have covered them exactly where they need to be covered.

Restrict access to your XML sitemaps – Quite frankly, there are a LOT of rogue robots out there. They gobble up XML files like locusts on a field of grain. You can protect your XML sitemaps from the rogue robots by using .htaccess files to restrict which search tools have access to your crawling lists. Let the major search engines see what you have on your site and tell the rogue robots to go fly a kite.

I have often used XML Sitemaps.com to create small XML sitemaps, but you may need to develop a custom solution or find another tool for truly large sites.