Short answer. The person you sold the other portion to. The Civil Code lets a seller whose remaining land is left isolated demand a right of way over the part he conveyed — but unlike the buyer in the reverse situation, the seller must pay indemnity for it.
What the law says
if it is the land of the grantor that becomes isolated, he may demand a right of way after paying a indemnity
Civil Code, Article 653 — When the Grantor's Land Is Isolated. Read the full provision →
The law looks first at who created the enclosure
When land is cut in two by a sale, exchange or partition and one part is left without an outlet, the Civil Code does not send the isolated owner out to the nearest neighbour. It sends him to the other party to that same transaction. The reason is simple fairness: the enclosure was produced by the parties' own act, so they must solve it between themselves rather than burden a stranger's property. Article 653 covers the half of that rule that catches sellers by surprise — the case where it is the grantor's land, the part he kept, that ends up isolated.
You can demand the way, but you must pay for it
Article 653 says the grantor may demand a right of way after paying a indemnity. The right exists, so the buyer cannot simply refuse and leave you walled in; but it is not free. That is the deliberate difference from the buyer's position. A buyer who is left enclosed by the same transaction gets his outlet without indemnity, because the seller sold him a landlocked parcel. A seller who kept the enclosed portion carved up his own property and therefore pays. The indemnity reflects the value of the strip taken and the damage the passage causes to the burdened land.
The exception for donations
The article adds one qualification: the donor shall not be liable for indemnity. Where the split came about through a gift rather than a sale, the law does not make the donor compensate the donee for the passage he needs across the donated land. If your situation involves a transfer to a child or relative documented as a donation rather than a sale, the money side of the answer changes even though the underlying right of way does not. Check what the deed actually says it is, because families often describe as a sale what the document records as a donation, or the reverse.
Practical points before you demand anything
Read the deed first. Many deeds of sale already reserve an access road, which settles the matter without litigation. If none was reserved, put the demand in writing, propose a specific route and offer a figure for the indemnity — the passage should be at the point least prejudicial to the burdened land. Have the survey plan ready, since disputes about right of way are usually really disputes about where the strip runs and how wide it is. A right of way granted this way is an easement that should be annotated on the titles so it survives a later sale. Outcomes depend on the documents; none can be promised in advance.