As per What are Salesforce ID's composed of?, the 4th and 5th character of a Salesforce ID can be used to identify which pod/server the record was created on. Note that due to events like pod migrations the pod identifier may no longer match the actual pod the record is hosted on.
What are the current mappings?
As per Converting the Salesforce Pod Id to ServerUrl/Name, I started the mapping process from the source code of Workbench.
That list however is largely out of date. As a community resource I've compiled the list from orgs I have access to. Please feel free to fill in gaps as you can.
NA
30 NA1 (no longer exists) 40 NA2 50 NA3 60 NA4 70 NA5 80 NA6 A0 NA7 C0 NA8 E0 NA9 F0 NA10 G0 NA11 U0 NA12 a0 NA13 d0 NA14 i0 NA15 j0 NA16 o0 NA17 12 NA18 13 NA19 14 NA20 t0 NA21 15 NA22 16 NA23 1a NA24 31 NA25 32 NA26 34 NA29 36 NA30 37 NA31 38 NA32 41 NA35 0L NA39 46 NA40 27 NA41 2A NA42 0G NA43 0M NA44 5A NA49 0V NA51 0W NA52 f2 NA53 f4 NA59 0e NA62 0g NA64 0h NA65 1B NA68 1K NA75 1M NA77 1N NA78 1O NA79 1R NA82 1S NA83 1T NA84 1W NA87 AP
10 APO 90 AP1 28 AP2 6F AP4 7F AP5 0I AP7 0o AP8 2v AP15 2w AP16 EU
20 EU0 D0 EU1 b0 EU2 w0 EU3 CS
T0 tapp0 S0 CS1 R0 CS2 Q0 CS3 P0 CS4 O0 CS5 N0 CS6 M0 CS7 L0 CS8 K0 CS9 J0 CS10 Z0 CS11 V0 CS12 W0 CS13 c0 CS14 e0 CS15 f0 CS16 g0 CS17 11 CS18 29 CS19 m0 CS20 ?? CS21 17 CS22 18 CS23 19 CS24 1b CS25 22 CS27 23 CS28 n0 CS30 p0 CS31 2f CS34 2g CS35 54 CS40 55 CS41 63 CS43 7A CS44 8A CS45 9A CS46 - an internal instance that Salesforce uses for performance stress testing, quality checks on upcoming releases, and similar activities ?? CS47 ?? CS48 0u CS49 3B CS50 4B CS51 5B CS52 0j CS53 0S CS54 0k CS57 0l CS58 2C CS59 3C CS60 4C CS61 5C CS62 6C CS63 0m CS65 0v CS66 0n CS67 1D CS68 3D CS70 4D CS71 5D CS72 0w CS75 9D CS76 0t CS77 1k CS78 1h CS79 25 CS80 8E CS87 9E CS88 1F CS90 2F CS91 4F CS93 0R CS94 0x CS95 1g CS96 1x CS100 1j CS102 3I CS123 3K CS125 7Z CS174 7a CS189 Pre-Release
B0 Pre-Release: GS0 Other
xx Not affiliated with a Pod