域名的解析IP为何不固定

import requests
import dns.resolver
import dns.update
import dns.query
import dns.tsigkeyring

# 获取当前公网IP地址
def get_current_ip():
    response = requests.get('https://api.ipify.org')
    return response.text

# 更新DNS记录
def update_dns_record(domain, new_ip):
    keyring = dns.tsigkeyring.from_text({
        'your_key_name': 'your_key_secret'
    })
    update = dns.update.Update(domain, keyring=keyring)
    update.replace('@', 300, 'A', new_ip)
    response = dns.query.tcp(update, 'your_dns_server')
    return response

# 主逻辑
def main():
    domain = 'example.com'
    current_ip = get_current_ip()
    print(f'Current IP: {current_ip}')

    # 查询当前DNS记录
    answers = dns.resolver.resolve(domain, 'A')
    current_dns_ip = answers[0].address
    print(f'Current DNS IP: {current_dns_ip}')

    # 如果IP地址发生变化，更新DNS记录
    if current_ip != current_dns_ip:
        update_dns_record(domain, current_ip)
        print('DNS record updated successfully.')
    else:
        print('DNS record is already up to date.')

if __name__ == '__main__':
    main()