Excellent Matt Cutts link from John Conde:

For several years Google has used something called “host crowding,” which means that Google will show up to two results from each hostname/subdomain of a domain name. That approach works very well to show 1-2 results from a subdomain, but we did hear complaints that for some types of searches (e.g. esoteric or long-tail searches), Google could return a search page with lots of results all from one domain. In the last few weeks we changed our algorithms to make that less likely to happen in the future.

 

This change doesn’t apply across the board; if a particular domain is really relevant, we may still return several results from that domain. For example, with a search query like [ibm] the user probably likes/wants to see several results from ibm.com. Note that this is a pretty subtle change, and it doesn’t affect a majority of our queries.

The "host crowding" has been documented before; it means that many hits from the 'same' domain may be grouped and suppressed. This extends to subdomains x.example.com and subfolders example.com/x equally, but does not extend to results at example-a.com and example-b.com.

And most importantly, in context of the question:

A subdomain can be useful to separate out content that is completely different. Google uses subdomains for distinct products such news.google.com or maps.google.com, for example

Which is basically our goal. Well, that, and cutting the gordian knot of 25 (and counting) impossible naming situations.

Another good analysis -- in conclusion, multiple domains vs subdomains is a wash.

Use subdomains when you have very disparate content that you feel searchers would feel relevant. For instance, videos are very different from articles, so it may make sense to separate those into subdomains. If you have a travel site, it may make sense to use subdomains to categorize cities. Someone searching for a particular resort may want to see the various locations that resort is located in.

 

In particular, look at the use of a single domain versus subdomains versus separate domains from a business perspective. What makes the most sense for your users? If you have completely different business units, they may appear more credible as individual subdomains.

Excellent Matt Cutts link from John Conde:

For several years Google has used something called “host crowding,” which means that Google will show up to two results from each hostname/subdomain of a domain name. That approach works very well to show 1-2 results from a subdomain, but we did hear complaints that for some types of searches (e.g. esoteric or long-tail searches), Google could return a search page with lots of results all from one domain. In the last few weeks we changed our algorithms to make that less likely to happen in the future.

This change doesn’t apply across the board; if a particular domain is really relevant, we may still return several results from that domain. For example, with a search query like [ibm] the user probably likes/wants to see several results from ibm.com. Note that this is a pretty subtle change, and it doesn’t affect a majority of our queries.

The "host crowding" has been documented before; it means that many hits from the 'same' domain may be grouped and suppressed. This extends to subdomains x.example.com and subfolders example.com/x equally, but does not extend to results at example-a.com and example-b.com.

And most importantly, in context of the question:

A subdomain can be useful to separate out content that is completely different. Google uses subdomains for distinct products such news.google.com or maps.google.com, for example

Which is basically our goal. Well, that, and cutting the gordian knot of 25 (and counting) impossible naming situations.

Another good analysis -- in conclusion, multiple domains vs subdomains is a wash.

Use subdomains when you have very disparate content that you feel searchers would feel relevant. For instance, videos are very different from articles, so it may make sense to separate those into subdomains. If you have a travel site, it may make sense to use subdomains to categorize cities. Someone searching for a particular resort may want to see the various locations that resort is located in.

In particular, look at the use of a single domain versus subdomains versus separate domains from a business perspective. What makes the most sense for your users? If you have completely different business units, they may appear more credible as individual subdomains.

Excellent Matt Cutts link from John Conde:

For several years Google has used something called “host crowding,” which means that Google will show up to two results from each hostname/subdomain of a domain name. That approach works very well to show 1-2 results from a subdomain, but we did hear complaints that for some types of searches (e.g. esoteric or long-tail searches), Google could return a search page with lots of results all from one domain. In the last few weeks we changed our algorithms to make that less likely to happen in the future.

This change doesn’t apply across the board; if a particular domain is really relevant, we may still return several results from that domain. For example, with a search query like [ibm] the user probably likes/wants to see several results from ibm.com. Note that this is a pretty subtle change, and it doesn’t affect a majority of our queries.

The "host crowding" has been documented before; it means that many hits from the 'same' domain may be grouped and suppressed. This extends to subdomains x.example.com and subfolders example.com/x equally, but does not extend to results at example-a.com and example-b.com.

And most importantly, in context of the question:

A subdomain can be useful to separate out content that is completely different. Google uses subdomains for distinct products such news.google.com or maps.google.com, for example

Which is basically our goal. Well, that, and cutting the gordian knot of 25 (and counting) impossible naming situations.

Excellent Matt Cutts link from John Conde:

For several years Google has used something called “host crowding,” which means that Google will show up to two results from each hostname/subdomain of a domain name. That approach works very well to show 1-2 results from a subdomain, but we did hear complaints that for some types of searches (e.g. esoteric or long-tail searches), Google could return a search page with lots of results all from one domain. In the last few weeks we changed our algorithms to make that less likely to happen in the future.

This change doesn’t apply across the board; if a particular domain is really relevant, we may still return several results from that domain. For example, with a search query like [ibm] the user probably likes/wants to see several results from ibm.com. Note that this is a pretty subtle change, and it doesn’t affect a majority of our queries.

The "host crowding" has been documented before; it means that many hits from the 'same' domain may be grouped and suppressed. This extends to subdomains x.example.com and subfolders example.com/x equally, but does not extend to results at example-a.com and example-b.com.

And most importantly, in context of the question:

A subdomain can be useful to separate out content that is completely different. Google uses subdomains for distinct products such news.google.com or maps.google.com, for example

Which is basically our goal. Well, that, and cutting the gordian knot of 25 (and counting) impossible naming situations.

Another good analysis -- in conclusion, multiple domains vs subdomains is a wash.

Use subdomains when you have very disparate content that you feel searchers would feel relevant. For instance, videos are very different from articles, so it may make sense to separate those into subdomains. If you have a travel site, it may make sense to use subdomains to categorize cities. Someone searching for a particular resort may want to see the various locations that resort is located in.

In particular, look at the use of a single domain versus subdomains versus separate domains from a business perspective. What makes the most sense for your users? If you have completely different business units, they may appear more credible as individual subdomains.