PageSpeed: Minimize DNS lookups (deprecated)
Reducing the number of unique hostnames from which resources are served cuts down on the number of DNS resolutions that the browser has to make, and therefore, RTT delays.
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Before a browser can establish a network connection to a web server, it must resolve the DNS name of the web server to an IP address. Since DNS resolutions can be cached by the client's browser and operating system, if a valid record is still available in the client's cache, there is no latency introduced. However, if the client needs to perform a DNS lookup over the network, the latency can vary greatly depending on the proximity of a DNS name server that can provide a valid response. All ISPs have DNS servers which cache name-IP mappings from authoritative name servers; however, if the caching DNS server's record has expired, and needs to be refreshed, it may need to traverse several nodes in the DNS serving hierarchy, sometimes around the globe, to find an authoritative server. If the DNS resolvers are under load, they can queue DNS resolution requests, which further adds to the latency. In other words, in theory, DNS resolution takes 1 RTT to complete, but in practice, the latency can vary significantly due to DNS resolver queuing delays. It's therefore important to reduce DNS lookups more than any other kinds of requests.
The validity of a DNS record is determined by the time-to-live (TTL) value set by its primary authoritative server; many network administrators set the TTL to very low (between 5 minutes and 24 hours) to allow for quick updates in case network traffic needs to be shifted around. (However, many DNS caches, including browsers, are "TTL disobeyers" and keep the cached record for longer than instructed by the origin server, up to 30 minutes in some cases.) There are a number of ways to mitigate DNS lookup time - such as increasing your DNS records' time-to-live settting, minimizing CNAME records (which require additional lookups), replicating your name servers in multiple regions, and so on - but these go beyond the scope of web application developmeent, and may not be feasible given your site's network traffic management requirements.
Instead, the best way to limit DNS-lookup latency from your application is to minimize the number of different DNS lookups that the client needs to make, especially lookups that delay the initial loading of the page. The way to do that is to minimize the number of different hostnames from which resources need to be downloaded. However, because there are benefits from using multiple hostnames to induce parallel downloads, this depends somewhat on the number of resources served per page. The optimal number is somewhere between 1 and 5 hosts (1 main host plus 4 hosts on which to parallelize cacheable resources). As a rule of thumb, you shouldn't use more than 1 host for fewer than 6 resources; fewer than 2 resources on a single host is especially wasteful. It should never be necessary to use more than 5 hosts (not counting hosts serving resources over which you have no control, such as ads).