ARIN was issued the IPv4 address blocks 173 /8 and 174 /8 by the IANA on 4 February 2008.
ARIN will be issuing /20 and shorter prefixes from these blocks to customers within the next 2 weeks. Network operators may wish to adjust any filters in place accordingly.
For informational purposes, a list of ARIN's currently administered IP blocks can be found at:
Director, Registration Services
American Registry for Internet Numbers (ARIN)