I went to my zoneedit.com, changed mydomain.com to point to a different IP. But changes haven't taken affect yet.
Is this because my ISP DNS is caching?
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Yes. Your ISP is almost certainly caching DNS settings for some period of time. They are supposed to refresh the records when the TTL expires. Unfortunately there are a large number of ISPs that seem to ignore TTLs all together in their DNS caching schemes. If you happen to be on one of those ISPs it could be hours or even days before they respect the new records, even if you have a very low TTL set.
There are two things here. One is the time it takes for your domain registrar to place the DNS entries you've provided through some sort of web form / management panel on the registrars DNS servers. This 'update' process usually occurs within 15 minutes.
Two there is the TTL field of each RR (Registry Record), which basically says: if someone asked about domain.com before, cache the domain.com -> IP address RR for TTL seconds (cache so I don't have to ask an authoritative server again / every time some host asks about domain.com). TTL values vary, usually they are set to about 1 day == 86400 seconds.
You can actually check if the DNS server of your ISP cached the result, by asking the ISP DNS server explicitly, using the commands:
nslookup domain.com ns1.isp.com dig domain.com @ns1.isp.com
The TTL in the response field will indicate for how much time the RR is cached on the server.
Yes. Your domain's DNS record specifies a TTL ("time to live") value, which directs client computers (and ISP nameservers) to cache results for a specified length of time before asking again. Default values of 24-48 hours are typical.
Here's one good practice for making future transitions go more smoothly: go in a few days ahead of your cut-over date and change the TTL to something very short, e.g. 300 seconds. When you configure the new IP address, you can set it back to 24 hours. From your perspective, the chief benefit of a long TTL is that your site's visitors will experience the performance benefit of cached DNS queries. It also lightens the load on your domain's nameservers.
In light of your current situation, you may want to explore the possibility of continuing to mirror your content for a few days at the old address (if your site is static) or configuring the HTTP server at your domain's old IP address to refer requests to the new IP address (if your site is dynamic).
Your ISP runs a DNS server that is caching DNS entries for as long as you tell it to.
If you control a DNS record, one aspect of that record is the "TTL" which tells DNS servers (and properly behaving clients) how long it should cache that value after it gets it from the authoritative DNS server.
Please note -- there is a tremendous distinction between your ISP and your ISP's DNS server. There is nothing forcing you to use your ISP's DNS server. You can run a recursive server yourself, you can use google's DNS servers, you can use OpenDNS.
Also, if you're expecting to change your DNS records often, and you want those updates to take effect quickly, you should reduce the TTL of your records. You'll see more load on your DNS server, but at the same time, your DNS chages will be seen faster.
It takes up to 72 hours for zone record updates to complete propagating worldwide. And yes your ISP caches DNS queries
Edit: There are a number of online tools you can use to track the progress of the update. Check out this one.