This is covered in Epic Level Basics.
Epic Attack Bonus
Similarly, the character’s base attack bonus does not increase after character level reaches 20th. However, the character does receive a cumulative +1 epic bonus on all attacks at every odd-numbered level beyond 20th, as shown on Table: Epic Save and Epic Attack Bonuses. Any time a feat, prestige class, or other rule refers to your base attack bonus (except for gaining additional attacks), use the sum of your base attack bonus and epic attack bonus.
What does it mean to you?
You continue to gain Base Attack Bonus until your character class is level 20. Any character class level starting at level 21 nets you Epic Attack Bonus rather than Base Attack Bonus.
Base Attack Bonus gained from racial hit dice do not count towards your 20 class levels of Base Attack Bonus. That is why you can see creatures, such as dragons, have a Base Attack Bonus of +25 or higher without having a single class level. Base Attack Bonus from racial hit dice all gets added to the Base Attack Bonus granted by class levels. However, you can't gain more than 4 attacks a round with Base Attack Bonus no matter what your Base Attack Bonus is.
If I have 10 levels of Barbarian, my Base Attack Bonus would be +10. If I gain 10 Levels of Sorcerer, my Base Attack Bonus would be +15. I am now 20th level. I would gain Epic Attack Bonus rather than Base Attack Bonus upon gaining level 21 and so on.
The only way to increase my Base Attack Bonus in Epic Levels, is to somehow gain Racial Hit Dice. One way of doing that is becoming a lycanthrope. You would gain Base Attack Bonus with your animal hit dice.
So, my 10th level Barbarian/10th level Sorcerer would has a Base Attack Bonus of +15 (three attacks a round). If I somehow became a werebear (brown bear), I would gain +4 to my Base Attack Bonus from my animal hit dice. That brings my Base Attack Bonus to +19, and consequently giving me my 4th and final attack per round.
When you look at prestige classes in the epic level handbook, you will notice there are no Base Attack Bonuses listed under any of them. That is due to not gaining Base Attack bonus with class levels from level 21 on up. If you take a non epic prestige class, or any other class for that matter, you still gain Epic Attack Bonus because your class levels are epic (more than 20).
Looking at the red dragon (great wyrm) you can see he has 40 hit dice and a +40 Base Attack Bonus. If he were to gain 20 levels of fighter, his Base Attack Bonus would become +60. However, his iterative attacks would only be +60/+55/+50/+45 since he can still only have 4 attacks a round from Base Attack Bonus.
Don't confuse that with natural attacks, they are a different matter altogether.
Determinging Iterative Attacks
Iterative attacks are in "lump" of 5. You don't gain a second attack until your Base Attack Bonus is +6. That is why you see +6/+1. You do not gain a third attack until your Base Attack Bonus is +11. That is why you see +11/+6/+1. You do not gain a fourth attack until your Base Attack bonus is +16. That is why you see +16/+11/+6/+1. When adding Base Attack Bonus from multiple classes, only use the bigger number all the way on the left.
For example, if I take 10 levels of fighter, I would have +10/+5. If I then take 10 levels of barbarian, I would have +10/+5 from barbarian. When determinging my Base Attack Bonus, I just add the two +10's together. That gives me my +20 Base Attack Bonus. Remember the -5 rule... my attacks would then be +20/+15/+10/+5. If I added +10/+5 to +10/+5 I would have +20/+10... which is not accurate.
Don't worry about numbers that come after a / to the right of the big number. Just worry about that one big number on the left - for math purposes.