Difference between 15 digit and 18 digit ID in Salesforce

• 15 digit case-sensitive version 
• 15 digit ID is referenced in the UI
• 18 digit case-insensitive 
• 18 digit id is referenced through in the API
• Why do we need 18 digit IDs, because many Legacy system [ex: excel sheets] work with Case insensitive IDs
• The id displayed by a report is the 15-digit case-sensitive id. The ids returned by web services api are 18-digit id. For updates web service accept both the 15-digit and 18-digit ids.

BreakDown of 15 Characters - 

As there is a combination of lower and upper case letters the casing of the 15 character Id has significance. E.g. 50130000000014c is a different ID from 50130000000014C.

Within a 15 character Id the breakdown is:

• First 3 characters - The first 3 characters are the key prefix that identify the object type. There are some exceptions to this where multiple objects all share the same key prefix! 
• There are a number of fixed key prefixes that are common across all of Salesforce .  Custom objects in managed packages can have a different keyprefix in each installed org.(Cross check once)
• The 4th and 5th characters - Reserved. Currently used for the instance id / POD Identifier. Indicates which pod/instance the record was created on.
• 6th character - Reserved. Will be 0 until such time that Salesforce has a need for it. See more
• Remaining 9 characters - basically a really big number. Like 62^9 big.

To this you can add an optional 3 character suffix that will make the Id unique case-insensitive.

 This is useful when working with programs that can't maintain the case of the ID (E.g. Excel VLookup).

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