CLARIFICATION IN IKEV2 AUTHENTCATION PAHSE 2

hi,
iam implementing ikev2, now ii have completed the phase 1 of the ikev2
, and now iam about to startthe authentication phase, in
authintication phase ,iam having one doubt .
section 2.15 (rfc 4306) authenticatin of IKE_SA ,
here he has mentioned that AUTH = prf(prf(shared secret ,"KEY pad for
IKEV2"),)
here what is that shared secret ,whether the shared secret generated
from phase 1 or any other
2: and whether the text pad is fixed or any thing we have to take.
if we take shared secret of phase 1 , i am having one doubt i.e how to
identify other user is valid
means any intruder who can actively paricipate in pahse 1 and get
authenticated , because we are not communicated with ip number (and
when NAT is there all will communiate with same ip).
so please some body clarify my doubt.
is there any other shared secret key is there, and here he mentioned
about digital signature ,
what are the rules to do that

Thanks & regards
RAMASWAMY BM
GlobalEdgesoftware Ltd
BANGALORE