What constitutes a “neighborhood” among the billions and billions of sites on the Internet? Just as real-world neighborhoods consists of communities linked together by common networks of streets, roads, fences, boundaries, highways, railroads, and pathways, online neighborhoods, too, are intricate systems of links.

And just as living next door to a derelict building, crack house, or noisy neighbor would reflect badly on your own real estate values (no matter how scrupulously you maintained your own property), virtual proximity to bad neighbors has much the same effect on search engine rankings.

Here’s what Google has to say about SEO real estate:

So, what’s a bad Web neighborhood? Any site that uses underhanded or unethical tactics to try to increase its site’s rankings or mislead its visitors can be classified as a bad neighbor.

These dirty tricks include the following:

For a more in-depth discussion of these misdeeds, see Part IX, “Don’t Be Evil.”

All these infractions can result in a site being penalized or quarantined by Google—as well as the other search engines. Link to these bad apples, and you’re guilty, at least in part by association. Like real-world bad neighborhoods, online danger zones are often recognized by a propensity toward drugs, sex for sale, gambling, and other illicit-bordering-on-illegal activities.

When in doubt as to whether a site to which you’re interested in linking is considered a bad neighborhood, a simple search engine check can help reveal how the search engines themselves regard the property. Enter this protocol: site:companyurlhere. If the domain doesn’t show up in the search results, it’s probably because the site is question has been penalized. Because you’re judged by the company you keep (by search engines as well as visitors to your own site), it’s probably best not to link.

Don’t lose too much sleep over this if you’ve got an above-board, ethical linking policy. The odd bad link isn’t going to torpedo all your search engine optimization efforts and knock you out of the search ballpark for all time. What search engines, specifically Google, are most concerned with are linking patterns. As Google’s Adam Lasnik put it on a WebmasterWorld.com discussion board: “Our algorithms are tuned to look for patterns of ‘egregious’ linking behavior—both on individual sites and in the aggregate.”

A big part of determining whether a neighborhood is good is not only learning more about your neighbors, but also keeping your own yard clean. Is the lawn clean and tidy? Is the grass mowed, and are the flowers blooming? Or is the front porch littered with the rusting shell of a discarded refrigerator next to a torn and sagging sofa?

In other words, it’s important to conduct routine maintenance on your own links. Too many links to 404 pages (that is, web pages or sites that have moved, or no longer exist) are not only contextually irrelevant and not helpful to visitors, but also can mount in number over time and, unchecked, create problems. So, although not linking to sites of ill repute should be common sense, it’s also important to keep your own home tidy and regularly check links, updating or eliminating them if necessary.

Keeping your own site neat and tidy makes the Web a better neighborhood for everybody, and it certainly won’t hurt your search rankings, either.