Serial clustering uses two or more network devices connected in series via a local and/or wide-area network to provide additional capacity when network traffic exceeds the processing capabilities of a single network device. When a first network device reaches its capacity limit, any excess network traffic beyond that limit is passed through the first network device unchanged. A network device connected in series with the first network device intercepts and will process the excess network traffic provided that it has sufficient processing capacity. Additional network devices can process remaining network traffic in a similar manner until all of the excess network traffic has been processed or until there are no more additional network devices. Network devices may use rules to determine how to handle network traffic. Rules may be based on the attributes of received network packets, attributes of the network device, or attributes of the network.