Match and extract if found

[menator01]

Here is a long way of doing it. Much room for optimizing.

def compare(arg1, arg2):
    list1 = []
    list2 = []
    lastlist = []
    with open(arg1, 'r') as file:
        lines = file.readlines()
        for line in lines:
            if ':' not in line:
                list1.append(line.strip())

    with open(arg2, 'r') as file2:
        lines = file2.readlines()
        for line in lines:
            if ':' not in line:
                list2.append(line.strip())

    for line in list2:
        if line not in list1:
            lastlist.append(line)
    return lastlist

print(compare('IP1.txt', 'IP2.txt'))

Output:
['192.168.0.799', '192.168.0.900', '192.168.0.1000', '192.168.0.83']