The modified EUI-64 portion of the address is the host portion of the IPv6 address. So it's the last 64 bits.
It uses the MAC address of the interface to derive this host portion. Let's take a look at our MAC address.
R1#show int fa0/0 | in bia Hardware is Gt96k FE, address is c001.0d20.0000 (bia c001.0d20.0000)
So if we write out the first byte of this address in binary we have 1100 0000. Now we count from left to right to find the 7th bit. Once we find the 7th bit, we need to invert it. Doing this we get 1100 0010. Converting this back to hex, we get c201.0d20.000. Now since the MAC addrss is only 48 bits and the host portion of our IPv6 address is 64 bits, we need to add some padding of FF FE in the middle of the MAC address. Once we go that we get c201:0dff:fe20:0000. From here, we can use our rules of eliminating the leading 0s and we get c201:dff:fe20:0.
R1#show ipv6 interface brief FastEthernet0/0 [up/up] FE80:1:1:11::1 2001:1:1:11::1 FastEthernet0/1 [administratively down/down] Loopback0 [up/up] FE80::C201:DFF:FE20:0
2000:1:1:1:C201:DFF:FE20:0 <----That's how the router arrived at the host portion of our address.
