with open('show_arp.txt', 'r', newline='') as f:
    # Reading file in list (each line is list element)
    contents = f.read().splitlines()

    # The argument newline='' is necessary only in Windows

    # Eliminating the first item from the list (files header)
    contents = contents[1:]

    # Empty list that stores tuples like (ip, mac)
    ip_mac = list()

    # Iterating over the list(file contents) line by line
    for line in contents:
        ip = line.split()[1]     ## Getting the IP
        mac = line.split()[3]    ## Getting the MAC

        # Adding the tuple to the ip_mac list
        ip_mac.append((ip,mac))

    print(ip_mac)   # [('192.168.122.10', 'aabb.cc00.0200'), ('192.168.122.20', 'aabb.cc00.0100'), ('192.168.122.30', 'aabb.cc00.0300')]

## show_arp.txt contents:
#Protocol  Address          Age (min)  Hardware Addr   Type   Interface
#Internet  192.168.122.10          -   aabb.cc00.0200  ARPA   Ethernet0/0
#Internet  192.168.122.20          0   aabb.cc00.0100  ARPA   Ethernet0/0
#Internet  192.168.122.30          0   aabb.cc00.0300  ARPA   Ethernet0/0