i am trying to add loopback interface 0 and asisgn it IP address here is issue
2620Router(config-if)# ip address 172.16.0.0 255.55.0.0
Bad mask 0xFF370000 for address 172.16.0.0
How can i fix this?
your IP didn't work because:
1) it was not a host IP address but a network or subnet IP
2) your mask was incorrect: 255.55.0.0 it should have been 255.255.0.0 because in a mask you must have contiguous bits
which is not the case in 55( 00110111) and the message was giving you your wrong mask in hex representation
0xFF370000 is 255.55.0.0 in dotted decimal.
Provided point 1 was corrected you could have put any valid mask for a subnet in which the IP was included like a /24 for example. But it's best practice and in some cases mandatory like in MPLS VPNs to give loopbacks a /32 mask.
172.16.0.0/16 is a network/subnet You need to assign a host address to the interface
ip address 172.16.1.1 255.255.255.255