The IP addressed
18.104.22.168 (in addition to
22.214.171.124) is now used for Google's DNS server. How did they obtain this IP?
Specifically, what has to happen for an IP to be routed to a different location.
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It looks like they purchased and/or leased some address space out of Level 3's 126.96.36.199/8 block. This is a standard thing - a company with a large IP block such as a /8 (e.g. Level3's 188.8.131.52-184.108.40.206) will allocate blocks out to other companies. Level 3's had the 220.127.116.11/8 netblock for a while - since 1992 according to WHOIS. Below is an output from ARIN's WHOIS system:
Once these addresses have been registered as allocated out to another entity by Level3, Google can now advertise these addresses out of their infrastructure.
There's a bit of magic that happens behind that which would take a bit of time to explain. The Internet works by having a lot of disparate networks from different entities connect together to form the global network that we call the Internet. They use a protocol called BGP to advertise that they either own a block of IP addresses or that they have a path to get to them. Other entities use this advertisement to know where to route stuff destined for those addresses.
A lot of these entities (be it universities, companies, long-haul providers like telcos) also perform filtering of these advertisements as a way to keep entities honest about their advertisements. As an example, Joe Shmoe, Inc. couldn't advertise that they are the destination for Yahoo's network blocks because their upstream providers will only allow them to advertise the network blocks that are owned by Joe Shmoe, Inc.
The Internet's routing architecture is actually a lot more precarious than most people realize. It's built on a lot of gentlemanly agreements and implicit trust. It does occasionally happen that someone is not performing correct route filtering on a customer link and the customer will, either acccidentally or maliciously, advertise that they are the destination for networks that they don't own. This can, and has, cause outages for people and systems that are logically close (from an Internet standpoint) to the malicious advertisement.
Hope this helps.