• else will execute, if break was not used to exit the loop

DATA = """
127.0.0.1       localhost
127.0.0.1       mycomputer
172.16.0.1      example.com
192.168.0.1     example.edu example.org
10.0.0.1        example.net
255.255.255.255 broadcasthost
::1             localhost
"""
DNS = []

for line in DATA.splitlines():
    if not line:
        continue

    ip, *hostnames = line.split()
    # ip == '192.168.0.1'
    # hostnames == ['example.edu', 'example.org']

    for record in DNS:
        if record['ip'] == ip:
            record['hostnames'].update(hostnames)
            break
    else:
        DNS.append({
            'hostnames': set(hostnames),
            'ip': ip,
        })

print(DNS)
# [
#   {'ip': '127.0.0.1', 'hostnames': {'astromatt', 'localhost'}},
#   {'ip': '10.13.37.1', 'hostnames': {'esa.int', 'nasa.gov'}},
#   {'ip': '255.255.255.255', 'hostnames': {'broadcasthost'}},
#   {'ip': '::1', 'hostnames': {'localhost'}},
# ]