If you have both Intranet sites and a public facing portal, it is usually recommended to host them separately in their own farms instead of sharing the same farm. Some may argue that these two types of site can actually be hosted on the same farm as long as they are separate web applications using different application pools and even separate instances of the same service applications. However, there still are factors to be considered:

• Security – The servers of Internet-facing website are close to external access. It is a recommended security practice to separate servers hosting intranet from those serving internet-facing websites. In this case, intranet farms servers could be placed in an internal network zone with no internet access.

• Performance optimization – Intranet sites and internet-facing web site have different usage patterns. Internet-sites are primarily read-intensive, while intranet sites have more user updates. The configuration for these two types of environment are different. For example, in database server hosting content databases, the transaction log files of write-intensive databases should be put in a faster drive than that of the data files. For read-intensive databases, the configuration is quite the other way.

• Scalability – The growth pattern and scaling requirement is usually different between intranet websites and internet websites. Take the sizing of one of my customers’ SharePoint farm as an example, the intranet portal has a content database of more than 50 GBs while the total size of each of the internet web applications is only around 1 GB. The user traffic growth pattern may be different as well. The intranet traffic is basically predictable based on the growth of internal users. That of the internet farm may change quite drastically if the website becomes more and more popular.
• Maintainability – The maintenance schedule is usually different between internal intranet and internet-facing sites. Farm maintenance task often involves, solution deployment, IIS reset and even server reboot. Separating sites into two farms will ensure maintenance on Intranet farm does not impact the internet-facing websites. What is more, internet-facing sites have the most stringent SLA. Correspondingly, a dedicated high availability and maintenance schedule should be implemented.